Ask around, and you'll get all kinds of answers: Exhale when lifting, inhale while lowering holds it hard during the lift and breathes out with effort.
Research indicates that there are specific situations when holding your breath for a short period of time can pay off big in the weight room.
Many beginners and a few experienced lifters stop breathing during repetitive, low-intensity lifts, either because they think it'll make them stronger. An extended Holding manoeuvre like this can cause a dangerous spike in blood pressure, bursting blood vessels in your eyes and forehead, causing headaches and temporary vision disturbances. It could also cause you to faint on the spot, which has its own set of risks.
Act natural, when lifting sub-maximal loads, your normal exhalation-inhalation patterns are usually sufficient, or perhaps a longer breath phase with a short breath-hold at the most difficult portion of a lift.
If you utilize isometric contractions, pressure may build up high if you hold back your breath. In this case, more rapid breathing patterns can help—your body will let you know when. Hypertensive and cardiac patients should be encouraged to breathe through pursed lips and nose even when lifting heavy loads. For them, the Holding breath is a big no-no.
Careful instruction as to the technique of a given exercise will automatically result in the body responding with the optimal muscle recruitment strategy throughout the duration of the movement."
This applies to breathing as well. Focus on proper form, movement and breath naturally.