Try tightening the straps. Bra straps tend to stretch over time and the occasional tightening is needed for the perfect fit.
Another common reason for a slipping strap is that the bra band is too big. As the bra band increases in size, the strap placement changes and will be set too far out on the shoulder.
If you have narrow or sloped shoulders, look for a style that has straps that sit closer together or a racerback.