That's a tough question. So far all I've done is research dresses online so when I do go try stuff on, I have an idea of what I like. However after I buy my dress, the best thing to do would be stop looking at other dresses so I don't have those second-guessing feelings.
If you found something on ebay you really like, and it's not super expensive maybe just buy it and say it's for a TTD session or something. Or for an AHR if you are having one..