take a friend with you who has some idea about bikes (especially older bikes): good and bad brands, and also how to work on them.

Since spending a few years of obsessing about building up frames I got for cheap, I learned a lot--from friends! They often helped guide me through a deal (or talked me out of a bad one), or gave me suggestions on what i needed to do.

And figure out what you want in the bike--what will you use it for, etc. You really don't _need_ a new and expensive bike to start out with, but you also really don't want to get a bike that is a bad fit, unreliable, expensive to repair, or will need a lot of repair/upkeep, or will not be able to do what you want (i.e. if you commute, eyelets to put a rack on are really handy!)