Dan-
This can be a royal PITA. Most people try and take pliers to them and pull the shoes out, the problem with that method is that when you pull against the holder it can tend to expand the pad, thus making in much harder to get out. The method that I have found to work well is to put the brake pad in a vice and squeeze it down tightly, this will keep it from "expanding", then remove the holder from the pad rather than the pad from the holder.
Sean Flores
San Francisco, CA