When you use a brush with Water-Based Paints, clean up is usually pretty easy. The hardest part to clean is paint that works its way up at the top of the brush. If you wet your brush with water before painting, less paint will enter that part because the water got their first.
Step 1: Wet It
The water got up here first, so it will help keep out paint.
Step 2: Heard It on Home Time
I believe I saw this trick on Home Time by experts, so it can't be too bad.