I use a brush cleaning product from Home Depot. This link shows a blue can, but mine from a few years ago is in a yellow can.

Here is a link that might be useful: Brush cleaner

You can try soaking it in some alcohol over night and if that doesn't work some lacquer thinner. The only drawback to this is that after you buy the lacquer thinner you may have already spent enough money that you could have just bought a new paint brush. If you are going to soak it in lacquer thinner though, be sure to leave that outside or it will stink up your house.

thanks for the help.