I haven't done any online projects recently, but for some years I used to work pretty much exclusively on projects from rentacoder.com (now vworker.com).
The way I got into it was by starting bidding low on small jobs, getting good feedback, and progressively moving onto larger jobs. You'll find that the people willing to pay a decent amount on these websites also want experience and good reviews.
Once you have the reputation to even be considered, you need to make sure you bid on the right projects. That means finding projects that don't have a huge number of bids, and projects which match your previous experience. You need a portfolio. If you have spare time, spend it working on something which you can show off to prospective bidders. I'm pretty sure a little javascript asteroids clone I wrote 5 years back got me more work than any other reasons I gave people to hire me.
It also helps to concentrate on projects which are the latest big craze - when I was working, this was javascript. Not many people knew how to use it properly, so there were fewer bidders and you could charge higher prices. Of course, everybody "knows" javascript now days - I imagine phone apps is where it is at.
However you approach it, don't be discouraged when you don't win projects. It takes a while to get into the game. And regardless of how well you do, remember that you'd still make more money by working for locals (which is why I quit). Unless you enjoy it, theres probably better ways of making money.
Good luck!