The STL Check modifier is also another helpful tool, if you didn't know about it already. It selects problem faces or edges automatically, depending on the problem (doubled, flipped, open, etc.) so you can easily find what needs fixing.
Good point Eric, I'd forgotten about that. The only thing to note there is that it switches you to edit mesh (or at least it does in the version I'm using. 9 I think). Not a big deal, but worth noting in case you need to be in edit poly and forget to switch back ;) Interesting trivia: STL is short for STereoLithography,…