The comments section also contains some good suggestions. After wiping your brush well on a rag or paper towel, you can use either Goop hand cleaner, baby oil or safflower oil from the grocery store to remove the residual paint. Then soap & water (regular bar soap or Murphy's Oil Soap) will finish the job. Maybe top it off with Master's or Winsor-Newton Brush Cleaner.
There is also a post by Larry Seiler on WetCanvas! where he describes how to melt down Ivory soap. He uses it as an inexpensive alternative for cleaning brushes in the classroom.