Time-Life has a series of books called "The Home Improvement Library," most likely found at your public library. The volume "Special Purpose Rooms" has a section with many good ideas for sound-proofing.
I also suggest that you look into acoustics in general. Covering styrofoam -even the "fire resistant kind- with at least 1/2" of sheetrock is an excellent idea. Covering the exposed face of the wall with fiberglass batts (faced is OK) will also help the acoustics within the room.
The biggest help for acoustics within the room will be to avoid right angles; a rectangular room is simply awful!
