Minimize stress as much as you can. Are they too warm? Are they too cool? Is there plenty of room or are they a bit crowded? Is the brooder in a calm, quiet spot; or is it in a busy and noisy place?
All of these things are potential causes of stress, which will then lead to pasty butt. Most of the time, pasty butt is caused by chicks being overheated by heat lamps in small plastic container brooders. A big mistake new chick owners make is keeping the whole brooder at a constant 90+ degrees, which is much too warm. They only need one warm zone, the rest of the brooder should be significantly cooler, or even room temp. That can be hard to achieve in a small brooder with a heat lamp.
Molasses! It acts as a Laxative and thins their poop so it does not paste. I had 100 broilers that were getting it really bad and it cleaned them up. Just add it to their water. I sprinkled cayenne pepper in their food as well.