When I took some basic voice lessons (and I should have continued them!) the instructor had me do the following every day:

1. While standing, slowly inhale through your nose; you should feel your stomach distending; 2. When you feel you cannot take in another molecule of air, SLOWLY EXHALE through your teeth, making a "ssss" sound until you have exhaled as much as you can; 3. Repeat several times, trying to maintain a constant and steady stream of air (no big rushes of air out, or wavering/quavering)and draw it out as long as you can. Gradually, you will be able to exhale with much more control, and be able to carry a phrase to the end or hold a note consistently...... which reminds me I should practice this!

Its a quick and easy way to warm up the voice too, without doing any damage - also relaxes you (like breathing through a panic attack!)

Pixie