1). Make time for it. Usually to find great pieces, you have to hunt through the racks. If you're in a rush, you won't give yourself the time to really dig.
2). Don't go in with an agenda. For me when I thrift I just like to go and see what I can find. I don't go with a shopping list, cause that's when nothing is every right.
I have a couple, a Nicole Lee hand bag, I got in college, and most recently a pair of nude Steve Madden pumps.