Never hold your breath when you lift a heavy weight.  This could cause dizziness, fainting and at the worst a heart attack, a hernia or a stroke.  When you strain and hold your breath, your chest is put under so much pressure that the blood in your veins can't return to your heart.  Apply this principle to when you're moving furniture or whatever else.  Always breathe!

Keeping a healthy flow of oxygen in your bloodstream is one of the biggest favors you can do for your body. It improves your body's ability to function and helps you to lose weight.  If you make a habit of breathing the right way, you'll greatly maximize the benefits you get from weight training.

Breathe normally through each exercise and don't hold your breath or huff and puff from too much straining.  In addition, follow this simple guideline:

Breathe out (exhale) as you lift, pull or push against the weight.  Remember, "Exhale when you Exert".

Breathe in (inhale) as you relax.

For example, if you're doing bicep curls, breathe OUT as you LIFT the weight and breathe IN as you LOWER it.  This will get easier the more you do it.