[dibbs]

Recording studio foam is sick but its pretty spendy for the good stuff. Google it online or head down to your local Guitar center or high endish music store.

The cheap alternative used by many is eggcrates positioned with the "egg" side inwards towards the room. I dont know what your budget is, but if it isnt big, find something to cram in the gap (old sheets or something), then secure a board with eggcrates on it on each side of the wall. Wont look too good, but probably get the job done.

Recording studio foam is dope, and will look much nicer, but it is pretty spendy most of the time.