My old band used backing tracks for keys and harmony backing vocals. Worked well, but you may have to keep in mind we used a click track as well on one channel to keep the drummer in check. If you aint too fussed run them from an ipod type thing, but the more backing tracks you have, you may have to lash out for a multi track recorder, so you can balance levels. As for the actual tracks, get a keyboard player to play any lines....