So presented to you below, in no particular order, is a list of tips and tricks that will help you become a worthy opponent. When you pick up a weapon, always check how much ammo is left. Many players will take a gun from a fallen opponent or comrade only to realize in the middle of a firefight that it has no ammo in it. Needless to say, this can be a deadly mistake. Even if you've only fired one shot out of the magazine, reload.
Ammo is generally pretty cheap, and there is no excuse for going into combat without full reserves.
As a general rule of thumb, if you have less than half a magazine after an encounter, reload. Getting caught with 2 rounds left in your assault rifle will probably get you killed unless you are very accurate or very lucky. This is especially true with a shotgun. You never know when one extra shell will save your hide. This tactic can usually mean the difference between being the winner or being a corpse.