Cover when reloading
When you are reloading you are at your most vulnerable. Different weapons take different amounts of time to reload. The shotguns -- the M3 and XM1014 -- have the most rapid reloading mechanism: You can reload them one round at a time, and fire between rounds. As for the rest of the weapons, they must be reloaded one magazine at a time. It's best to take cover when reloading -- don't continue your assault. When reloading, take cover and avoid getting the enemy's attention.

Don't habitually reload after a kill
Many single player games that are superficially like Counter-Strike encourage players to reload almost instinctually after they score a kill, since the next enemy will be around the corner waiting for them. In Counter-Strike this a bad idea, since your enemies will often travel in groups. Instead, conserve ammunition by only using as much as you need to kill your target, and then wait until a lull in the fighting before reloading. Suppress the urge to immediately reload after every kill. Instead wait until you know there are no enemies nearby.