Enamel thinner.
Just go to your hardware or auto supply store and get a gallon can of it, ensure that it is an enamel thinner. There is no need to get a specific brand name just for cleanup. Some guys swear by having a 'general' thinner for every day cleanup, and then a heavier Lacquer thinner for the thorough cleaning jobs once a week/day/month/whatever. Experiment a bit and see what works best for you.
I also recommend scouting out a small rubbermaid container that is sturdy and large enough to fit your brush in when broken down. This allows you to soak the brush in a sealed container (less fumes).
As always, plenty of ventilation when working with this stuff (respirator if you have it), but that should be as elementary to model working as "don't cut towards your hand with that razor knife".
Lastly... and this is an enforcement of what was stated in all threads about this stuff: Consider your health. Look into getting a paint-grade respirator. You can get them at any good hardware store or places that carry this sort of chemical. Do a search here in the forums, I've posted everything on mine that I bought a while back.