I found a decent article on how to mine more out of your previous work but unfortunately I can only find a hideous-looking version online that's been cached so here is the link, but I'm excerpting some of the best tips from "Spring Cleaning for Your Writing," by Shaunna Privratsky:

"Dust off some of your old manuscripts, even those that have been accepted. If you hold the reprint rights, resubmit them and try to double or even triple your sales. If an article of mine has only sold once, I consider it still active.

"Look at your rejections, too. Much as writers hate them, they can be a stepping stone to future sales. Are there some editors who expressed interest, but you didn't follow up because they rejected your first submission? Go back and think of a new query or submit something better."