Contrary to popular belief, the hottest new resource for underpaid freelancers isn't MediaBistro or Gawker — it's jack-of-all-blogs Gothamist. From their "Ask Gothamist" feature:

I'm a freelance writer and usually work for bigger media outlets, so getting paid is no problem. Last January, however, I did some work for a private production company; five months later I have yet to be paid. I've invoiced a number of times (and spoken to the guy who hired me on the phone), and he keeps saying "I can't pay you until I get paid." I'm getting really frustrated! I'm reluctant to take legal action, because the invoice is only for $1,000. Short of busting knee-caps or stalking him on my BMX bike, how do I get my money?

The first step, in Gawker's opinion, would be to seek this kind of advice from outlets that actually pay their writers.

Freelance Does Not Mean "Working For Free" [Gothamist]