What type of pickup / mic and amp are you using? If you can fill out your TalkBass profile at "My TalkBass", it's real helpful to me & to others so they can look at your background info.
For one thing, standing right in front of your amp can sometimes cause problems. If it does, try to position yourself where your speaker's direct line of fire can't "see" your bass.
Actually, if I stand close enough so the amp is firing right into my DB, sometimes I can get that "controlled feedback" thing goin', kinda like Jack Casady with Jefferson Airplane & Hot Tuna.
