China bristle paintbrush stiff from oil based paint and varnish
I've had some success with recovering a gummed-up brush using a paint brush comb.  Soften the bristles by soaking them in thinner for a while, then use the comb to brush out loosened paint; repeat, probably several times.  If the brush is really bad, you may just be better off buying a new brush.

It's hard to get a brush completely clean, so to help prevent this, I have a large glass jar with some mineral spirits in the bottom.  I hang the brush from the top of the jar with wires so that the bristles are submerged (but not resting on the bottom).  Seal up the jar and the brush is OK to use next time.