I have several "tricks"that I use
1) 4-shot (obviously) Which, in case you didn't know, is three shots to the chest, one to the head.
2) Throw a frag and right when it explodes shoot once in the head for an instant kill
3) Never go up against an AR close range (obvious)
4) Best combos are with: AR, Plasma Pistol, Shotgun, Sword, or Sniper (In my opinion)
5) Sometimes it's best to find a nice little area to sit back and snipe people off with the BR.
6) When up against a Banshee, most people will run away from a couple BR shots.
7) Don't plan on going up against a Warthog with the BR unless your team is coordinating an attack on it or your a safe distance away.
8) When against a Sniper, keep shooting at him while moving close, that way he can't scope in on you and he'll probably run for cover, making him a considerably easier target.
9) Don't complain about the different spread from Halo 2, people won't care.
Well that's the most I can come up with off the top of my head.