I'm a bassist, so I know your pain.
A lot of places sell something called acoustic foam, which is basically that black foam on walls at studios, etc. It can be expensive, but you can get the same basic effect with those egg-crate style foam mattress pads. The same thing can be done with thick carpeting or tapestries, and it also helps to put up curtains or something around your kit to act as a sound baffle, even a relatively thin curtain will help cut down noise. I had a corner of the room with some egg-crate foam stuck up on the wall and some thick blankets hanging from the ceiling, and they say it really helped cut down on noise (I wouldn't know, I was on the inside)