There are many elements required to being successful in Combat, many are obvious to the rookie commander and some are very nuanced and may take some time to learn and use to the best effect. The key as in all such situations is to have mastered all tactical knowledge available to you, but be able to apply in flexibly to the situation in hand.
The first element needed is the ability to understand and control the Battlescape itself, by learning what it will and won't show you in any given situation and mastering the interface.
The next step is to know your resources, their capabilities and their limitations so that you can apply this knowledge to use the most appropriate tool for the job. The two most important parts of this are to know your soldiers and their abilities, and also the equipment you have available and what tactical usage it can be put to.
The next element is to be prepared to use the terrain you encounter to give your soldiers cover where possible, and to identify potential hiding spots and ambush locations. It is also essential you can minimize the effects of adverse conditions such as darkness.
The next stage is to understand your enemy, make sure you know the alien races, their strengths and weaknesses, their weaponry and their special abilities like Mind Control and what these can potentially do to your forces.
Next thing to keep in mind at all times is your mission, in many cases it will be a simple case of recovering or assaulting a UFO, in which case you can clear the area with all due care. When responding to Alien terror attacks or defending your base your actions will have to be modified accordingly.
Once you have a full understanding of the battlefield and all the arrayed forces, then you need to ensure you can control your forces in such a way to maximize their strengths and minimize their weaknesses. From Initial Deployment to Sweeping the Battlescape and onwards you will need to find ways to eliminate the myriad advantages the enemy holds over you in many missions you will have to complete.
You will also need to acquire key information about the way the battlescape works so that you are not caught by surprise, and can make the most of any opportunities you get. This encompasses elements such as learning how to scout, maintaining morale, the best use of smoke and explosives, as well as Line of Sight and Firing Accuracy. In addition knowledge of how soldiers improve themselves as they complete missions, and how to keep them alive if they take Fatal Wounds.
All through history the most able general knows what his troops are capable of.
With this in mind, I rename all of my troops with their attributes! Bravery score, reaction, accuracy, strength and then the number of kills they have. How many times has the cpu given a rocket launcher to someone that has str 20? It helps you deploy snipers, reaction fire troops, useless, er, I mean medics etc. Also helps you save, from the line of fire your "chosen" troops.