Health. Health packs are the simplest but most necessary power-up in Halo-- the only time you won't need it after a firefight is if you either killed them before they saw you or died by their hand. Know where the health is on each map, and use it. It can make all the difference.

Overshield. The overshield is useful for that extra edge in combat. It doesn't make you invulnerable, though. The overshield in multiplayer slowly drains away on its own, so make sure you don't go up against a superior enemy thinking you have better shields, only to find the overshield has bled away. It provides an extra pain for a sniper, and you appear no differently. However, overcharged plasma shots will drain it just as well, and a close-range shotgun blast will still kill with an overshield. Just cause you have an overshield shouldn't make you overconfident.

Active Camouflage. Invisibility can be fun, but remember that if the enemy is looking for you, they may still be able to see through the cloak. Also, you still show up on radar, as long as you are going above 50% speed. Getting active camo when sniping can also be useful, but don't let it bleed away while you are zoomed in and die thinking you were still cloaked.