What I've learned from games:
-Always have a way out.
-If you make yourself too much of a hassle, they'll probably just give up (I'm dead serious).
-Your value isn't based on how many you can kill, but on how many you can keep occupied.
-If you're on the run, turn around once you pass a corner for a free kill.
-The mission is infinitely more important than killing that one guy skipping through the open field with a bullseye on his helmet (even if you have a silencer).
-If killing an enemy means putting yourself in danger, they're not worth it.
-The hierarchy is "soldier < tank < anyone with a rocket launcher".
-Only make yourself visible if you're holding a line.
-When everything is fail, and the enemy is moments from victory, you might as well be epic.