I just stumbled on this and thought it was a really good overview of querying advice: http://talcottnotch.net/index.php/queries There's nothing groundbreaking there for anyone already familiar with querying, but I think it's a great little guide for anyone writing their first query.
You may have seen it called 'giving comps' (comparisons). Some agents really like them. I didn't do it, because I didn't like the risk of comparing myself to an author/book the agent disliked. But it can work--it's a matter of luck, like most things in querying!