Several advanced troop-training methods are possible in X-COM, especially once your troops are capable of mind-controlling aliens. The basic technique is to disarm the aliens, march them to a location of your choice, and have your soldiers fire at them. This technique is commonly called a firing squad.
These techniques should ideally only be performed once a mission field is completely secured -- i.e., all aliens have been located, disarmed, and "rounded up".
Once you have a few psionically-able soldiers armed with Psi-Amps, you can Mind Control alien units and have them do your bidding. Disarm the aliens (using the alien inventory exploit) and place them in front of the soldiers you wish to train. Hit the End Turn button so that the aliens will regain control of themselves (shooting X-COM-controlled units will not gain you experience), and fire at will.
Keep in mind that a squad comprised of several psionically-strong and skilled soldiers will need little further training of the type detailed in this article, as such soldiers can manipulate the alien forces at will.
Aliens with psionic abilities (Ethereals and Sectoid Commanders/Leaders) can allow additional specialized training techniques. Their psionic panicking attacks can be used to train soldiers' Bravery stat, and X-COM units under alien control can be used for target practice to gain experience.
Special precautions are needed when using psionic aliens for training, however. All soldiers should be disarmed (this includes grenades!), or equipped only with weapons that cannot harm a soldier in a Power Suit: standard Pistols or Rifles, or Autocannon-HE against Flying Suits.
Combat actions and experience
Depending on how many combat "actions" your soldiers perform, their stats will improve by a variable amount at the end of a mission. With one action, their stats will improve 0-1 points; with three actions, 1-3 points, with six actions, 1-4 points, and with 11 or more, 2-6 points. See Experience for further details.
Stat increases are specific to the stat exercised: making reaction shots increases your Reactions; hitting an alien increases your Firing Accuracy, using a Psi-Amp increases your Psionic Skill; throwing objects increases your Throwing Accuracy; and not panicking despite low Morale increases your Bravery.
However, there are four "secondary stats" (Time Units, Energy, Health, and Strength) which may increase following a mission during which a soldier performs any action -- even just one. (Throwing Accuracy actions are excluded.) See Experience#Secondary Stats for full details.
All stats, both primary and secondary, have a "cap", above which they can never rise. See Experience#Regarding Caps for more details.
Stat-specific training methods
Reactions training (EU)
You can train your soldiers' Reactions stat by leaving an armed but non-mind-controlled alien within their field of view. If the alien moves, this will trigger reaction shots, thereby scoring your soldiers a Reactions "action". (See Reaction Training for more details.)
Often a disarmed alien will not move, or will not move enough to trigger reaction shots, especially from soldiers with a very low Reactions score. "Laser Pistol Gifts" is a very effective technique in this situation. Using the alien inventory exploit again, have the alien stand on top of a Laser Pistol and pick it up.
Make sure all soldiers within the alien's line of sight are wearing a Power Suit or Flying Suit and directly facing the alien. A Laser Pistol can do a maximum of 92 points of damage, which cannot penetrate the front armor of a suit -- but it can get through the side and back armor.
Now the alien will fire at your troops to no avail, using up all its TUs. Even a soldier with a miserable Reactions score will return fire upon the alien.
To optimize the number of actions your troops perform, arm the soldier(s) you are training with standard Pistols. Pistols are the weakest weapons in the game, and require the most shots to kill aliens -- giving your troops the most shots at an alien before it dies. (See Kill Modelling for more details.)
A pistol clip also holds 12 shots, which is a good de facto record-keeper: it approximates the 11 actions soldiers will need to perform to get maximum stat gains at the end of a mission. (See Experience for more details.) After a soldier empties the clip, reload the weapon and pass it to another soldier whose Reactions you wish to train.
One other alternative is to handle a standard Rifle. Though it is equal in accuracy but slightly stronger than the pistol, you can make use of its two-handedness to drop its base accuracy from 60% down to 48% by holding an item in the other hand. The lower odds reduces the chance of hitting the test subject. Remember not to kneel as well!
A soldier does not have to hit an alien to score a Reactions action -- merely taking a reaction shot will increase the actions counter by one. Occasionally the counter will increase without even taking a shot; see Note 3 at User_talk:MikeTheRed#Tips on Increasing Skills. (There are several other good tips on that page as well.) This latest information has led me to develop a new, possibly better way of training... the INCENDIERY ROCKET gift!!! Which will be on the same page as Laser Pistol Gifts, for now.
You can arm the alien with a standard pistol as well -- but a Laser Pistol is best as it does not require any ammo. A Laser Pistol in the hands of your troops is not recommended, as it will kill the aliens more quickly.
Reactions training (TFTD)
The training technique in TFTD is similar to that in EU, except that it is not possible to give aliens a weak weapon since their inventory is not accessible. The Lobsterman's melee attack can be used as a substitute.
Disarm the Lobsterman (using M.C.) and position it directly in front of an aquanaut in Magnetic Ion Armor. Have the other aquanauts hover overhead, facing the alien.
Often, the Lobsterman will perform five melee attacks on your aquanaut, then turn or walk one square. The melee attacks won't trigger reaction fire directly, but as soon as the alien moves, your aquanauts will unload their clips - even the ones with terrible reaction skills.
The Dart Guns will not harm the Lobsterman or your aquanaut. The Lobsterman's melee attack is will not harm your aquanaut as long as it's a frontal attack.
Sometimes the alien decides to walk around and/or attack from the side. In that case, M.C. it again and walk it back, or load your save game.
As long as the rest of your squad is floating overhead, the alien will not use Sonic Pulsers.
Firing Accuracy training
Any time one of your soldiers fires at and hits an alien unit, he or she scores a Firing Accuracy action. Like Reactions, 11 actions are the most needed to maximize stat gains -- so if during your Reactions training any of your soldiers manage to score 11 hits, they need no further training for that mission.
Each hit will score you an additional Accuracy action, even if you aim at one alien and hit another. Auto shots count as three actions, if all three shots hit. You will also score an Accuracy action for each alien caught in the area effect of an explosive round (including a Stun Bomb) -- and one explosion can strike multiple aliens. Explosions from grenades will score an Accuracy action for each alien hit as well (but not from Proximity Grenades). The last soldier to have thrown the grenade will be awarded the experience. If not thrown, it'll default to the unit that started on the equipment pile.
One devious way to train your soldiers' Firing Accuracy is to bring one or more psi-weak soldiers into battle wearing Power or Flying Suits. Psi-enabled aliens will often Mind Control such soldiers -- and hitting any unit controlled by the aliens will score you an action.
Your entire squad can then take turns shooting the hapless puppets with a light weapon, such as a standard Pistol or Rifle. A Laser Pistol can also be used, but be careful to only hit the victims' front armor, so as to spare them from injuries. A Laser Pistol should not be given to a unit that is psi-weak and might go berserk, or worse still, get mind-controlled.
This technique unfortunately cannot be used to train Reactions, as X-COM soldiers will never take reaction shots at another soldier, even one that is under alien control and actively shooting all the other soldiers. It does however combine well with Bravery training, detailed below.
Shooting aliens currently under X-COM mind control will not count as an action.
For a "cheap" way to safely train accuracy, also see ExploitsE#Milking_Alien_Bases
Psionic Skill training
Psionic Skill is the easiest stat to train. Soldiers can simply be kept in training in a Psionic Laboratory for several months (details here), or they can bring a Psi-Amp onto the battlefield and muck with aliens' minds to their hearts' content.
Since a lot of training methods hinge upon mind-controlling aliens in the first place, your most psi-adept soldiers will probably get battlefield Psi Skill training automatically. Furthermore, since a successful psi attack counts as three actions instead of one (one is still scored for an unsuccessful attack), it is a trivial matter to gain Psi Skill in combat. (Just be sure to not go past 255 actions -- 85 successful psi attacks per soldier -- as this will overflow the experience counter, making it start counting from zero again. Details here.)
Rigorous training through battle will raise your psi skill to 100 fairly quickly. To get it beyond that, you must train through the labs. Do not let it go beyond 255, or else you will have to start training from scratch again.
Either you gain 10 bravery, or 0, per mission. Your soldiers will only gain Bravery experience when their Morale drops below 50. Morale is lost when X-COM *controlled* units die -- since letting soldiers die is not a practical way to train, one way of doing this is to MC aliens, then shoot them while still under X-com control. Another way is to expose your troops to psi-enabled aliens. See Bravery#Improvement for full details.
If you have the resources to spare, a distastefully crude but effective method of artificially reducing morale for the purpose of bravery training is to hire a large contingent of throw-away rookies, gather them up and drop a high explosive amongst them and then let your trainees run around in a panic. Remember to take other precautions before attempting this, such as clearing the map and disarming the troops that are being trained in case they go berserk.
A TRULY outlandish but effective way of training bravery is through an exploit and bug. Exploiting Mind Control, you allow the Commander and Colonels to be zombified, once all Snakemen have been killed. (Make sure everyone is using flying armor) This will WRECK everyone else's morale, much worse than 10 rookies being killed. Don't mess up and end up not resurrecting your top troops! Alternatively, I have found a disgustingly easy technique, which can be used to train your entire squad's (-1 volunteer) bravery at once. Kill all aliens except for 1 Chryssalid. Now, feed one of your soldiers to the Chryssalid, while having everyone else in a safe spot, facing away from the alien/s. Next turn, 1 unit turns around, spots the aliens, mind control the zombie, turn around again. End turn. Turn around, spot aliens, mind control the zombie, turn around again. Repeat add infinitum. Result: Eventually, entire squad turns into chickens. Disadvantage: You will end up with a REALLY bad score for that mission, since it will count as having many many soldiers dying over and over.
Throwing Accuracy training
Throwing Accuracy is trivial to train: each time a soldier throws any object, they will get a Throwing Accuracy action. If you're positive to have killed the last alien, have your soldiers throw away all their gear before the turn ends. Unlike all the other stats, performing Throwing Accuracy actions will not trigger secondary stat increases.
Training methods not involving psionics
It is possible to use several of the above training techniques before your soldiers become psi-capable, although with greater difficulty and risk. However, there are still several relatively-safe techniques you can employ.
Aliens which have panicked and dropped their weapons are often still carrying an Alien Grenade, which they will use, so approach unarmed aliens with caution. However, grenades cannot be targeted at "mid-air" and will only damage units on the level where they explode, so soldiers hovering off the ground in Flying Suits can approach such aliens safely.
Aliens which have lost consciousness will drop all their weapons, including grenades. Aliens will never pick up objects, so once they regain consciousness (due to a treatment with Medi-Kit stimulants, perhaps?), they cannot harm X-COM units. Terror units with a built-in attack (primarily Chryssalids and Celatids) remain dangerous at all times, however. 4-space terror units -- Reaper, Sectopod, and Cyberdisc -- will never regain consciousness once stunned.
Ethereals and Sectoid Leaders/Commanders can still perform psi-attacks when disarmed, and are dangerous to engage in general except with soldiers known to be resistant to psionic attacks. Anti-psi tactics can lead to rich training opportunities here though, allowing relatively safe bravery, reaction and accuracy training, since alien Psi attacks do appear to draw reaction fire.
Silacoids, because of their slow speed and lack of ranged attacks, can also be used to safely train your troops. Be sure your troops keep at least 12 spaces away to avoid their attacks (or simply hover off the ground in a Flying Suit). Silacoids' high armor (50 on Experienced level or above) also makes them excellent for Firing Accuracy and Reactions training -- a Pistol will seldom damage them, allowing many soldiers to get in some training before it dies. Incendiary rounds are particularly effective for training on a live Silacoid, as they will not harm a Silacoid at all. Incendiary rounds will also never harm a soldier wearing Personal Armor or better -- an added benefit in the event of stray shots.
Aliens which have only a melee attack (Reapers, Chryssalids, and Silacoids) can also be attacked safely by any troops hovering in a Flying Suit (or by going up stairs and then destroying the stairs). If you can kill all the weapon-carrying aliens first, you can then have a good training session with the remaining terrorists.
As noted above, stunning weapon-carrying aliens will make them drop all their equipment. Wake them up with Medi-Kits, and let the now helpless aliens run around the place, drawing reaction fire and deliberate fire from X-Com. This allows training of Soldiers right from day 1, since you theoretically only need Stun Rods, and patience for the alien to wake up naturally. Possibly lots of patience or lots of Medi-Kits, since an alien can easily be at perhaps -100 Stun after a Stun Bomb hit, or perhaps -50 Stun after a Stun Rod hit. Since you can't stun or kill all the hostiles (otherwise the mission will end), you need to be on the look out for threats during the training, but you also may need to hold your fire to keep the 'training mission' going. (Once some unarmed aliens have woken up, you can stun the rest with impunity, and relax). For the very patient, this makes the Small Launcher possibly a powerful training tool. Rush small launcher research and bring plenty of pistols and pistol clips into combat.