Field Manual: Terrain
Firing over the crest of a hill, known as the reverse-slope position, exposes the minimum sight picture to the enemy and provides the greatest protection from return fire. Simply ducking into concealment protects the soldier from all but indirect-fire weaponry (grenades). Allow time to climb the hill before seeking out the enemy.
Soldiers should seek to overwatch from hill crests and move in the protection of valleys or next to slopes.
Movement up and down hills takes time, so a soldier on flat ground will always outpace a soldier on a slope.
Enemies hiding in valleys are best dealt with through grenades or other indirect fire.
Vegetation, especially in jungle or forest environments, provides a high degree of cover. Thick-trunked vegetation can often stop enemy fire temporarily, but will quickly be shredded by powerful weaponry.
Thick vegetation reduces sight lines and is a dangerous environment to move through. Scouts have to look all around them as each movement can reveal previously hidden areas. Overwatch and support teams will find trees blocking both direct and indirect fire and will have to move closer to the scouts to adequately support them. Paths through the vegetation are not always evident and soldiers may be forced to double back if their path becomes impassable.
Clearings should be exploited as rally points and kill zones. Soldiers concealed in vegetation on the edges of a clearing have the best combination of concealment and field of fire. High explosive can be used to clear vegetation as necessary.
[Jasonred] : Try experimenting with Incendiaries in this environment! Clear the landing area of enemies, retreat to the transport, fire a round or two of incendiaries, and wait for either the aliens to come to you as the fire catches them, or for the whole place to burn down and give visibility.
Artificial Structures: Buildings
Man made buildings provide good concealment and some degree of protection. However they are vulnerable to explosives (and energy weapons) and can be penetrated by high caliber weaponry. (See Destroying Terrain for more information.)
They are excellent for concealing movement and approach, but may equally conceal the enemy in their windows, doors, and corridors. In a free-fire zone, flatten these with explosive rounds to deny cover to the enemy.
Sniping from a high position or window minimizes exposure, but the soldier must be prepared for return fire and move to different cover if spotted.
Artificial Structures: UFO Hulls
UFO Outer Hulls, like our reinforced concrete bunkers, are generally impervious to man-portable weaponry. 
The commander has two choices: siege or storm.
Sieging has the benefit that time is on the commander's side, so forces can be arranged in good concealed positions with good fields of fire. All exits from the structure should be covered. Avoid placing units directly in line with exits or windows as these units will be the first ones attacked when the enemy ventures forth. The enemy will hide from a barrage but will hopefully become curious enough, if all is quiet and movement unopposed, to reveal themselves. This then allows your marksmen to engage them.
Sending forward a scout is risky, but can fool the enemy into revealing themselves to shoot at the presented target.
Sieges succeed through attrition of the enemy as they reveal themselves. A resolute enemy with adequate supplies, however, can choose to remain hidden indefinitely. So eventually the structure will have to be stormed.
Storming these structures is always hazardous, because no amount of "target softening" can guarantee them to be clear of enemy presence before the assault, EXCEPT in the case of Medium Scouts, in which case target softening can end up killing everything inside (and also destroying bounty from alien technology). The first soldiers in will always be in harm's way.  Single file entry is often required, so the assault team must enter quickly and spread out again to maximize effectiveness. Assault teams should carry weapons with high rate of fire and high stopping power. Armor is a must. Stun Rods can also be useful. Heavy Cannon can provide stopping power.
Training must emphasize prioritizing targets.
- a) An enemy facing the soldier and in line with the entryway, should be handed off to a following soldier or even to a support sniper outside the entryway. If neither of these are practical (due to blocked lines of fire) then engage at short range with burst fire (autofire).
- b) An enemy facing the soldier, but not in line with the entryway, must be engaged. At short range, use three-shot burst fire (autofire).
- c) If enough time remains to take cover and let a following soldier engage the enemy, but not enough time to both fire on the enemy and take cover, then take cover.
- d) An enemy facing away from the soldier should be engaged only if time permits, or if no other soldiers can move into position to take the shot. Stun rods are best used on this presentation of target.
- e) Enemies packed into a corridor are best engaged with burst fire. Use of explosives in such situations is unpredictable, given the close range and risk of secondary explosions.
Motion detectors are invaluable in gaining information about the enemy in such a structure. The soldier with the motion detector does not have to be a member of the assault team but should be moved up next to the structure.
-  Blaster Launchers will penetrate an outer hull. No other unmodified weapon has sufficient power. Heavy Plasma will eventually penetrate inner bulkheads.
-  SWAT teams get thick full length shields for this sort of thing. X-Com gets ... rookies.