I got to do this recently to someone who's relative new to the freelance field. Among the things I told her:
1) Be persistent. You will likely get rejected for most of what you apply for.
2) If someone wants to work with you, make absolutely certain there is no ambiguity in the payment. I did this once and ended up doing a $100 press release for free. That stunk.
3) Did I mention persistent?
4) For paying sites, look at how much they charge and the services offered and ask yourself if it is a worthwhile investment. If it's not, then you could keep the receipt and write it off at tax time.
5) Don't shy away from free sites like www.freelancewritinggigs.com. One absolutely wonderful, lucrative freelance relationship came from that site.
Here's something else I picked up while doing one of my all-time favorite freelance jobs of sports gathering at the San Antonio Express-News. The guy next to me was phonr interviewing a coach or player for an upcoming story. As he spoke, I learned three important things to do while interviewing somebody:
1) Be professional. Keep it business--especially if the person you're interviewing is a complete stranger.
2) Be polite.
3) Be to the point. Don't rabbit trail, and don't get into discussions that have nothing to do with the interview--unless it's a brief discussion that could possibly uncover a future story idea.