What are some good Halo 3 tips/tactics for Online play?

Aim for the head, not the chest, In deathmatches, keep on the move as much as possible, zig-zag to avoid being sniped, and practice hard.

sniper- aim 4 the head if you miss run so they dont find u
rocket launcher- before you fire jump and aim 4 them so 1 they have a less chance to hit u and 2 u have a bigger chance 4 succes
duck- when u c a red dot on ur radar duck it seems cowardly but what u r doing is getting off their radar waiting 4 them to pass and then get them from behind
stay together- not too together or 1 rocket kills the team
hide- if your team is getting mauled outside your door duck and hide (how i got the perfection award)
last but not least
always carry a plasma pistol with u (not recomended to inexperienced players) best gun you could carry

The tips depend on what weapons you use, like umbreonboy said, -
Sniper- aim for the head, if you are bad at sniping, try to get to a position where the enemy is coming your direction, and its easier
Rocket- avoid killing yourself (you shoot too close) and jump before shooting
BR- take advantage of the scope, try zooming in and shooting, if you are bad at scope, then just shoot the enemy
Grenades- use frag grenade for clusters of enemies or vehicles; plasma if you want to destroy aircraft or stick to someone/something; stick grenade is almost the same thing as plasma but I think its more effective... (?); and lastly, the firebomb grenade to burn them to h311 :D
Shotgun- always get close before shooting, whoever uses a shotgun from far away is stupid... no offense
Plasma Pistol- heres a cheap but effective way to kill, overcharge the pistol, and wait for an enemy, shoot them and their shields deplete, then quickly change over to another weapon, like BR or shotgun and shoot them, it will seriously damage or kill them.
Melee Weapons- Energy sword is the normal melee weapon, the Gravity Hammer- you can use it to play baseball...whack people so they go flying, or have fun with it :D
Thats all I can say right now... but you can go on gamefaqs.com or some other game site for more tips ty

Heres some sniper tips:
http://www.ehow.com/how_5742100_master-sniping-halo-3.html