Back when I was a 19 year-old assistant to a general contractor we did a lot of housepainting. He taught me to wash brushes by running water over them: first you let the water hit the bottom of the sink, and right there you sort of dab the brush hard against the sink. Then once the water runs out of it a bit clearer, you turn the brush upside down so the water is going straight into the bristles, and you sort of work the bristles with your other hand until the water runs out of it a bit more clearer still. Then you repeat that until the water running out of the brush is perfectly clear.
Then you take a hard wire brush and run it over the bristles, away from the handle of course, to remove the paint that's hardened there.
Then you rinse the brush well, spin or shake it until no more water flies out, smooth the bristles back into place, and then leave to air dry the rest of the way.
Takes about 5-15 minutes per brush if you're doing it right, but you can keep a good brush in very good shape for years that way.