I swear by Mona Lisa Pink Soap, available at most craft/art supply stores:
http://www.houstonart.com/pinksoap.asp
It\'s brush cleaner, preserver, and conditioner all in one product... \'nuff said. I wash my brushes under running room-temp water after every session, and then add a little of the stuff in afterwards to condition on occasion.
It\'s even restored shape and snap to my remaining 15+ year-old, crappy \"use it for everything including painting glue on\" brushes that I used to clean enamels from with mineral spirits - granted, there\'s not many hairs on those, but what\'s left looks and works great! I\'ve been using it on all my brushes, both Kolinsky sable and synthetic, for the nearly two years I\'ve been back at painting and it\'s amazing the amount of paint (even dried stuff) that you can drag out of the hairs and ferrule with Pink Soap from a brush that otherwise looks \"clean.\" (I generally try to not let paint dry on \'em and not let it work up to the ferrule, but stuff happens, ya know...)
I also generally add a few drops to my brush water, which seems to help as a preventative measure.
As an added bonus, the conditioner and/or magic ingredients in it leaves my brush-cleaning fingers nice and soft, for whatever that\'s worth.