One thing I find useful, which helps prevent disappointment in bidding, is just to do some basic market research. You cannot expect to get a $200 item for $20 every day of the week. Of course, it happens now and then, which is part of why we all love auctions!
eBay lets you search completed auctions. Use search terms to find completed auctions for whatever it is you are bidding on (this can be hard to do with extremely rare items, I know, but it's easy with mass-produced items). Find out what these items sold for in previous auctions, and you'll have a good idea what you can expect to pay. Of course, you may get lucky.
I always say, though, that I'm looking for a deal, not a steal. I want the seller to get a fair price too.