I found this great tip by Elizabeth on The Last Door Down The Hall (in the sidebar) on cleaning up filthy brushes and rubber stamps:
Do you have some crusty brushes you can't use laying around? or how about stamps with dried acrylic paint on them? A little Murphy's Oil soap will do wonders.
Soak your brushes in Murphy's Oil Soap. You will see the paint or gel medium start to dissolve. Rinse with cool water. Repeat if necessary.
For stamps pour soap into a flat tray or dish, set stamp in tray. Let soak. Use a tooth brush to gently dislodge paint and rinse. Good as new. Also, the soap helps keep your wood mount from drying out.
Note from me:If you can't find Murphy's Oil Soap Cleaner for wood, then try Pledge Soapy Cleaner for Wood
And here's a tip from The Happy Slob
Soak those gunky paintbrushes in pure white vinegar for a few minutes. The acid will help to loosen the bristles and make that old yucky paint far easier to remove once and for all.
Then, just wash the paintbrushes in warm soapy water, and you'll have a paintbrush that's as good as new!
Isn't this brilliant?