How do you find great comp titles?

When you are preparing a query for an agent or an acquiring editor, it can be a good idea to include one or two comparative titles in your pitch. Comp titles are books that are similar in some way to yours and they help agents and editors and publishing staff (like sales and marketing) know who is likely to buy and read your book, how well and where it will sell, and how best to promote it.

But, how do you know what titles are just the right comparisons?