First recommendation is to clean your brushes daily...I don't mean go all out with shampoo and conditioner and such, just have a paper towel on hand, and after you use the brush, squirt some brush cleanser (I use MAC) on the paper towel, and swirl the brush around until you get it clean. Then wipe gently on a clean, dry paper towel, and you should be good. That'll help prevent staining; the less time a product spends on your brush, the less chance it has to stain.
Second recommendation; dishwashing liquid. It'll pull the stain out, but also dries the crap out of your brush, so be prepared to give it a nice, intensive conditioning treatment when you're done washing. HTH!