Great advice, and some very impressive lifting (2x bodyweight for 3 sets of 30 is near unbelievable!), but the above information is wrong. Holding one's breath is called the valsalva maneuver, and is exactly what you should be doing when lifting a heavy object, and is, in fact, what the human body will do naturally.It is vital to remember to breath properly during each rep, whether doing squats or leg presses. Remember to exhale while you press the weight up. Never, ever, hold your breath and strain to make the rep. If you have to do this to lift the weight, stop and decrease the weight. Very bad things can happen otherwise, things that will irrevocably halt your career.
Imagine what you would do if you had to get a car rolling; shoulder down, big breath in and push until you're red in the face. This maneuver increases intrathoracic pressure, stabilizes the spine, and is the only to safely lift the weight. Most people will, to some extent or another, however, breathe, grunt, scream, moan, etc., during the end portion of the lift, but thoracic pressure is never lost.