Have you heard about the Stress Eraser? Its cost is a whopping $300. It's a biofeedback device that monitors your pulse and tells you when to breath.
Save your money and do the following instead:
Sit or lie in a comfortable position. Gently close your eyes. Notice your breath for the next 6 or so breaths. No manipulation, judgement, or change. Simply notice it.
Now guide the breath to a 1:1 ratio--so that the inhales and exhales are of the same length. Only do this as long as there is no strain. It may feel a bit "odd", but still comfortable. Take about 6 1:1 breaths.
Next slowly add one or more counts to each breath. That is, maintain the 1:1 ratio, but slow the breath some. Do this gradually, and only as long as you feel comfortable --NO strain at all. Continue to breath at this longer/slower 1:1 for another 6 breaths.
Finally begin to lengthen your exhales until you are at a 1:2 ratio (the exhales are twice as long as the inhales). As always, only do this as long as there is NO strain and a level of comfort still exists. Use the calming 1:2 ratio for 12 breaths.
Allow a natural breath to flow in and out--no counting or attempted ratios. Softly open your eyes.
Enjoy the peace and stress removal that the breath can bring!