1. If an enemy hides behind a good, solid cover, yet is suspiciously non-active and refrains from attacking your soldiers, you should be cautious - there's a good chance that he's a bait, and when you close in on his position to flank him you will reveal a group of hidden enemies, waiting to trap you.
2. Try to always have 1-2 Flashbang Grenades and at least a single Mimic Beacon at your disposal. The former will allow you to decrease the effectiveness of your enemies, while the latter gives you the chance to divert the attention of aliens off your troops, which is extremely valuable when you've pulled too many enemies on your squad.
3. Picking the proper type of ammo can significantly increase the effectiveness of your soldiers when dealing with certain types of enemies. You should be the most interested in A.P. Rounds, Bluescreen Rounds and Talon Rounds - more on them can be found in the chapter about ammunition.
4. From a certain moment in the game, enemies equipped with armor (yellow squares) and shields (red squares) will start appearing on the battlefield. As far as the latter goes, those are simply bonus health points, but the armor itself is a thing you should be extremely cautious about - each point of armor decreases the damage your soldiers deal by 1. It can be shredded with the help of grenades, and some of Grenadier's abilities.
5. If you aren't absolutely certain that there isn't any enemy around the corner (especially if you are not in conceal mode anymore), never, ever use sprint to move around the battlefield. By doing so you can easily fall into a group of enemies, completely unprepared.
6. The same goes for moving when almost all of your soldiers are done and out of action points and only a single soldier can do something - if that's the case, and if you're not short on the timer, you should always end your turn (either by getting into overwatch, or by hunkering down) and wait for the rest of your team. There's nothing worse than running into a group of enemies when your soldiers are done with their actions and can't do anything.
7. Don't ever leave your soldiers in solitude, without protection, even if they are behind a really good cover. A Viper can easily pull a soldier to himself across the whole screen, enemy reinforcements (or a disguised as a civilian Faceless) might appear very close to a lone soldier. This is especially important for sharpshooters, as this class is not the most effective one in close combat. Try to move your soldiers in pairs, so that you always have someone to help out.
8. If an enemy hides behind a good, solid cover, yet is suspiciously non-active and refrains from attacking your soldiers, you should be cautious - there's a good chance that he's a bait, and when you close in on his position to flank him you will reveal a group of hidden enemies, waiting to trap you.
9. Try not to clump up your entire team. A lot of enemies have an area of effect attack at their disposal, not to mention grenades - the more scattered your soldiers are, the less chances for an enemy hitting multiple of them with a single attack.
10. There will be numerous enemies on the battlefield each time, but there are some of them that should be taken care of as quickly as possible. Those are Sectoids (thanks to their mind-controlling abilities), Vipers (because of the fact that they can pull your soldier towards them and immobilize him), and Advent Stun Lancers (who can stun, or even make your soldier unconscious and unable to do anything for the rest of the mission).
11. Mind controlling ability, as well as zombie reanimation one which the Sectoid uses can be stopped by simply killing the enemy casting it, or by hitting him with a Flashbang.
12. To break free of the binding ability used by the Viper, all you have to do is to inflict a few points of damage to the enemy.
13. If any of your soldiers becomes unconscious, use the Specialist's Revival Protocol ability and the soldier will be back in action.