I use the same method as Bill - the long 1/4" bit as a guide hole. You just have to keep that baby super sharp to avoid wandering when you get to the angled roof sheet.
WD-40 works pretty good at getting the tar off of your bits if you don't want to pull the shingles before you start. Gasoline works even better, but the flamibility factor makes it impractical. Never clean a hot bit either way!
