|Use a nylon polyester bristle brush. Purdy and Linzer brushes found at Home Depot are good bang for the buck. Try to stay away from those $1.00 throw away brushes, because the bristles fall out everywhere. That's why they are $1.00!
You can not use a natural bristle brush with latex paint. The bristles will repel away from each other, and the brush will look like it exploded after it's in use for a while!