If you still see paint in the cup after you're done cleaning your brush, put a little acetone in the cup, let it soak for a couple of minutes, and then use an artists bristle brush to clean out the paint. Pulling the needle out, or back far enough to expose the entire cup, will help. Just cover the tip with your finger so the acetone doesn't drip out until you're done.
I also loosen up both pieces of the nozzle assembly, and back flush a cup full of acetone, to get at the paint that dries in the tiny air holes in the nozzle assembly.
Be sure to run some clean water, or airbrush cleaning solution, through the brush once you've finished with the acetone.
Back flushing with that will get all the acetone out, and help keep the brush ready for painting next time.