It's like what james said, but there are different methods. Insted of tapping my foot to keep the beat when i play, i play guitar when i sing. You have to know the song inside out to keep it in time, but sing to you guitar part don't play to your singing.
Hope I made sense.
Adrian
"You know, we do more than just sing and dance. We've got a brain, too."
Backstreet Boys