Besides the green glue page above, I HIGHLY recomennd checking out the Adio Visual Sciences (AVS) forums on Dedicated Theater Design and COnstruciton. Read the sticky Acoustical Treatments Master Thread. Be warned, it's really long but has GREAT info. Search the other threads for info on Green Glue.
I just (two weeks ago) finally had my basement theater drywalled with 2 layers of 5/8 rock and Green Glue. I also had them caulk all seams in the first layer. However, I also decoupled all of the framing using Resilent Sound Isolation Clips (RSIC).
Sounds like it's too late for the OP to decouple, but the drywall and GG will go LONG way towards helping and is probably the best bang for the buck.
Beyond that, you can make absorption panels out of Linacoustic (a fiberglass product used to line and quiet AC ducts) and an acoustically transparent material (if you can blow through it; it's AT).
Link to AVS Forum:
http://www.avsforum.com/avs-vb/forumdisplay.php?f=19Good luck!
