If you have advertised in the past on blogs, or have worked closely with someone has, do you have any tips for blogs that want to attract advertisers?

Naturally, we'd like to get some ourselves, but if we get zero advertising from here on out, we'll still be happy to blog away -- we've got cushy academic gigs that let us do this sort of thing as a hobby. But other bloggers might find this question even more important for them, since even a modest advertising income might make the difference between continuing and closing up shop. Any suggestions, whether for us or for others?