What I've found:
A) Never stop moving or looking around.
B) Always try to have a few escape routes for wherever you are in times of emergency.
C) Always try to figure out the only places they can be based on your radar and direction of fire. A lot of the time you can tell where your opponent is without even looking at them.
D) Don't be a d1ck. Most systems, especially Halo have a ranking system and people WILL report you if you're being r-tarded.
E) Remember it's a game. Practice, as always, is the only way to truly get better, and that only comes with time. Patience is key.