This one's a duh... Always make sure you have your controls setup so you can move easily and unawkwardly.
Also make sure the controls are not inverted or are inverted.Make sure that the look sensitivity is set to your way of playing. For example, a sniper might want a lower sensitvity so he can aim better, and a sword user might need high sensitivity to turn around faster.
It depends on what kind of game you're playing, but auto-look centering can be very crucial. I once played a game where there were only magnums and no sheilds. Auto-look centering made me win that so easy because the reticle would always be pointed at a person's head on flat ground. (Magnum headshots with no sheilds are instant death)
Don't stand around like a sitting duck. Always move around.
Multiplayer Tips
Crouch to not show up on radar.
When reloading, melee as soon as you can see the ammo refilled to reload faster.
If there is a sniper after you, when you jump, click crouch like a maniac to get not get shot so easily(During jumping, the Elite/Spartan has a perfect arc jump that is easy to snipe). Also, if a sniper is looking right at you, try to make the sniper waste ammo as much as possible by doing whatever you can.
Don't hide behind explosives : )
Shoot all explosives at the beggining of the game if your playing someone better than you. If your playing someone worse, explosives are your best friend.
Always be somewhere with some cover. Try not to be out in the open as much.
Look at the radar.
When sniping, turn around a few times to make sure no one is behind you.
Always use teamwork... No one can win by themselves on a 16 vs. 16 game...
TALK!!! Make sure that you and your team knows what you are doing. It doesn't matter if the other team can hear you, it's better than running around clueless.
During the start of the game, make sure you know which person is going to get what weapon, not weak weapons, but weapons like snipers, rockets, and swords.
Practice multiplayer tactics for Capture the Flag, Assault, Oddball, and others. Ex. Warthog pick up, Banshee riding, ect.
Don't always go rushing in like a fool. Wait for them to come to you. If you have lots of patience, you can ambush the other players better. For example, waiting behind a corner with a shotgun.
If your about to die with the rocket launcher, try to waste all the shots so the other team does not get it. If you are about to die, and you are very close to some enemies, shoot the ground. Also, if you are trapped against a guy with a rocket launcher, try to get very close to him/her. So if the guy shoots, you both die.
When you're about to die, try to throw a good weapon off a cliff. Make the other team wait for the respawn for the rocket, sniper, or sword.