I started off with using warm wet towels to get off pasty butt. I've found that just takes too much time. getting a glass bowl (glass holds heat better) and putting warm tap water in it is the fasted way to go. Then pick up the affected chick and hold them such that you can dunk their butts. I usually thread my thumb around their backs but under their wings. You have really good control over them and they look like they're sitting in a hot tub with their wings outstretched over the edges (my thumb). It's so cute!
Anyways, soak them that way for a few minutes and then you'll find that the paste can be worked off with a thumb and forefinger. You'll also remove less down this way, I've noticed.
Dan