It came as a surprise to me recently to learn that some people still use acetone to clean brushes after fibre-glassing. I must admit that I started off using it myself years ago, but it is very expensive if you are building as many boats as I was. A much cheaper alternative is just straight washing powder, as follows.

Use the container you have had the resin or gel in, tipping out any excess. Put in a spoonful or two of powder and add about two inches of warm to hot water. Work the brush up and down in the water, and round the sides of the container, inside and out. Work the brush with your hands, and let the fingers work up into the bristles. This first wash will get rid of 90% of the resin in the bowl, brush, and on your hands. Empty the container out and repeat the process. A third and fourth wash without any powder added, will complete the cleaning. You now also have a perfectly clean container to use again.

NOTE: The water must be hot, but not so hot that you can't work your hands in it. Not all soap powders are the same. Some tend not to dissolve properly. I have found best results with 'AJAX'.
Laurie Ford.

(1997 Update: 'Cold Power' or any of the more expensive better quality powders are all right, avoid the plain label types. Also, when working the brush in the water, keep bending the bristles over, up close to the handle, and see if you can see any 'colour' of resin or gel squeezing out. If you can, it needs more cleaning. Laurie Ford.)