The single largest barrier to entry for lucrative freelance contracts is ability to sell, and the second largest barrier is development of a reputation. Sites like oDesk remove both barriers, by standardizing and transactionalizing projects and by relying on a superficial reputation system.

The result is a race-to-the-bottom competition that will slash your rates and pit you against gamesmanship from suppliers that can't even deliver competently, but against which you'll have a hard time differentiating.

If you're considering freelancing over your summer vacation, you have more flexibility than almost all other freelancers. You should exploit that by developing your sales ability and your reputation, which will pay off handsomely when you leave school and it comes time to collect and negotiate offers.

One (of many) ways to do this:

1. Start writing articles (for your blog, but don't call it a blog). Start with your preferred technology stack and gradually branch out. Write case-study style about solving actual problems with Node or Angular or Rails or PHP or whatever it is you work best in. Work in specific languages with specific example, but make the focus of your articles solving actual problems and how you set about the process of generating those solutions.

2. Circulate articles on HN and Twitter; participate in related discussions.

3. Make it clear, constantly and regularly, that you're available for projects that are similar to what you've written about.

4. Optionally, build a "Github Resume" of the code that forms the basis of those articles, and link to it.

5. Start asking friends and acquaintances if they know people who have problems that could be solved well with software development; be forward once you find prospective clients, and, after establishing bona fides (for instance, suggest a 15 minute conversation about their problems and actually provide free advice), follow up by asking for the sale; you have nothing to lose --- "no" is a perfectly fine answer.

You'll do better long term executing this plan than you will on oDesk.