Adjust the length of the straps. Seems obvious, but over time your straps can loosen. Tighten them if need be, but not so much that they cut into your shoulders.
Yank the band of the bra down when you get dressed. It should rest just underneath your shoulder blades.
Trot over to a lingerie shop for a professional fitting. You may be wearing the wrong size. If your “girls” aren't completely filling their cups, the straps can easily slip.
Check the band size. If it is too large, or if you are using back extenders, the straps could be too far apart on your shoulders.
Try out different bra styles. Invest in some quality time in the fitting room with a variety of bras, preferably ones with straps set closer together. Try a racer-back or leotard-back bra. Sports bras also tend to stay in place no matter what you do.