There are lots of commercial devices to help but they can be expensive and often don't work very well.
I came up with this after seeing a similer device in a catalog for $80 and thought I could make it cheaper.
You will need:
1) A Sonicare or other sonic type toothbrush (You may be able to use other types of electric toothbrushes I just happened to have a Sonicare)
2) A single slat from a vinyl mini blind
3) A rubber band
Step 1: The Parts
Cut the slat about 8 inches longer than your toothbrush.