It really depends on how soundproofed you want it, but I assume like me you want it done cheaply. I would just cover the place in either studio foam wedgies or something, or the mlv (both on this site), and then for the low frequencies get some foam bass traps and set them around the area especially where bassy instruments will be located (i think those are on here, but if not try musiciansfriend.com). Now if you want it done really good, then I would do either all or most of the things that he recommends on this site for garage soundproofing (link on frontpage). I know you may know some of this stuff already, but I wrote like you didn't just incase.