Phyllis says, "My husband said he read this in an advise column. It really works! After painting, set your brushes in fabric softener for about 10 minutes (I have left mine even over night) and then clean as usual. This works for brushes used for all types of paint, even oil based paints.
I then tried the same thing with hair conditioner after using acrylic paint and it worked too. I was in a hurry and just dropped the brushes into the conditioner until I went back to the craft room... my brushes cleaned up faster and it worked well even after using gesso."