After using a brush with water-based paint, I've found that the quickest, best way to clean the brush is with a fork. Run a stream of water over the bristles while combing them with the tines of a regular dinner fork. The tines split apart the gummy paint that accumulates up higher in the brush, and lets the water quickly penetrate into the bristles so the paint rinses out quickly and thoroughly.
Once the water runs clear, give the brush a good shake to get rid of most of the water (I like to do this outside over the lawn), smooth the bristles into their proper shape, and leave it to air dry.
Once the water runs clear, give the brush a good shake to get rid of most of the water (I like to do this outside over the lawn), smooth the bristles into their proper shape, and leave it to air dry.