Number 1: Washable paint really is washable. We've washed it out of clothes, towels, furniture, dog fur, everything. Never had a problem.
Number 2: Set it up so it's okay to be messy. Put a big old beach towel down and train your child to stay on it when paint is being used--or else no more paint. If the rules are broken, paint goes away. Trust me when I say that after the first time, your child won't step off the towel. Stick to it and it works!
Number 3: Clean as you go. Stick a small (snack-sized) bowl of sudsy water along side the paintbrush water and show your child how to dip her fingers in it. She'll wash her own little hands as she goes. Little L. has just as much fun with the water as she does with the paint! (Evidence: See the patch of paper that we taped over part of her original work, where the paper got soaked and ripped. Not a problem!)
Number 4: Too much of a good thing can get ugly. Ever heard the saying, "A good Kindergarten teacher knows when to take the crayons away."? Step in the moment your child becomes a teeny bit bored, too silly, or destructive. Put new paper up just before he's quite done with the first painting. Aim for the "I want to do another one!" feeling, rather than the "I am completely totally utter DONE with this and I don't like it anymore!"