Well, I've tried to use rentacoder some time ago without success. Prices are ridiculously low. I have a friend though that is very active on rentacoder and making a living out of it.
From what I've understood it takes some time on these kind of sites to build a good score and reputation.
On development projects we prefer established projects and customers (2-3 developers working more than a year on a project etc.).
I would suggest creating your own company. If there is something you don't know how to do then you would need to outsource but you can offer a wide range of services that makes it hard for any individual or company to say no to you. Have a solid portfolio with unbeatable prices that gives you an edge over other competitors.
There is also 99designs.com, which I have used on two projects.
The problem is that it's a competition system - i.e. you enter your submission and the contest holder picks a winner from all the submissions. This means that you can enter a good few competitions without getting paid. However, I would imagine that the prizes are higher than the ones on the freelancing sites.
There is a large market from which a freelance writer can choose, and the sky is the limit as to what type of genre or how much work a freelance writer might have at any given time. A freelance writer whose specialty is fiction might dabble in poetry, short stories or e-books, or he might have an ongoing novel in the works. Many publishers will accept fiction but most freelance writers agree it is a much harder market to break into.