Anyone whose ever been to a great thrift shop knows that it can seem less like shopping and more like fighting in the Thunderdome. Thrift stores are usually locally run, which means they're pretty small, so it can be pretty crowded. They also have a limited number of dressing rooms, which means you can be waiting forever for the chance to try on your stash.
If you don't want to wait, show up to the store in a leotard. They're encompassing enough that you will be properly covered, but small enough that you can put anything on over it. Skip the line!