Proper breathing enhances your exercise where you are able to perform better repetitions before fatigue.  Recommended Breathing is however different when dealing with either light or heavy weights.

LIGHT WEIGHT TRAINING

Keep your breathing natural and avoid forcing it while doing exercises that use little weight effort for your personal capacity and for the muscle involved.

This style should also be used when you are working on very small muscles that put little demand on the cardio-respiratory system, such as the forearm.

HEAVY WEIGHT TRAINING

proper technique requires that you inhale through your nose and mouth at the same time (or only
through the mouth for short, quick breaths) during the first half or during the first third of the movement, and exhale through your mouth in the last half or the last third. The rest of the time you should hold your breath. 

In certain heavy exercises (e.g., squats or bench press) you can begin as you take the strain just before starting the movement, although this requires some practice. If you breathe in through the nose only, as you would do at rest, you will not produce sufficient air flow for the effort involved in weightlifting exercises.

Avoid eating,chewing or drinking while exercising as this hinders proper breathing.