Thanks to halo offical site

Over Shield:
The over shield can be useful, but it slowly drains away in multiplayer, so always be aware of your shield strength. You can surprise people sometimes with the over shield, as you don't appear any different to them. Beware of the plasma pistol over change however, as the over shield is no protection from it. Also remember that at close range the shotgun will still tear through it. The over shield can also be useful when you need to fall from extreme heights.

Health Pack:
Know where these are on each map and use them when you need to, as they will allow you to survive another fight. Make sure you have full health before committing to a face to face battle with another player, otherwise, chances are youll die first.

Active Camouflage:
Active camouflage with a sniper rifle is very good, as invisible players cannot be seen from a distance. Therefore, you are protected somewhat from other snipers -but be careful that you dont unknowingly let your active camo drain out while you are zoomed in and cant see your hands. People can see you on their motion trackers even with an invisibility, so try to move slowly, since you only appear on motion trackers at about 50% speed or faster. Shooting makes you visible again (but each weapon does this a little differently).