I know it seems funny to be reading tips about how to breathe, but the problem is that most of us do not breathe properly while lifting. In fact a lot of people actually hold their breath, which is problematic because it can cause your blood pressure to skyrocket.
So here is the deal. When performing exercises where you pull/curl, exhale as you pull/curl and inhale when the muscle is lengthening. For example, if you are curling a dumbbell, you would exhale as you curl up and inhale on the way back down. Easy right? It gets a little confusing when it comes to exercises where you press/push. So on pressing/pushing moves, exhale as you push/press and inhale when you return to the starting position. If you're performing a leg press, exhale as you press your legs and inhale as you return them back to the 90 degree angle.
In more simple terms, I like to think of it as exhaling when the move is hard, and inhaling when it's not as hard.
Fit's Tip: When in doubt, breathing normally is better than holding your breath.