I noticed that there was a recall for some air bag sensors... go check and see if your car is under that recall, and if so, get it replaced at a dealership. If not, you'll still want to get it inspected by an auto mechanic.
Slide your seat back some and see if that will make it go off. Sounds weird but there is a sensor under the driver seat and if your seat is too far ahead the bag would be in the wrong position to deploy on you. Try it. I have the same problem.