A bit more based on my extensive Odesk experience:
1. No less than 4.5 stars ratings; be aware that some/many contractors are sharing / using several accounts, so ratings may not always mean much.
2. Hire individuals, no agencies (very often, they switch contractor, depending on whomever is available).
3. At least 4/5 English speaking abilities ratings.
4. Ask for portfolio in bid, question them on the code through Skype; will take time but avoid disasters in the end.
5. Don't hire people you have a bad feeling about.
6. Request a keyword to be included in their bid (like "Mellinger"); if they don't write it, avoid them at all cost (shows attention to details/taking directions); also, only hire people whose bid makes sense and shows some kind of logic/intelligence..
7. Best candidates often found by searching for candidates that meet your requirements, then filter out (by language, scores, experience, rate, etc.) - but they are also the more
8. Be aware that Odesk technical skill "tests" don't mean much either (can be taken week after week until getting a perfect score; look at the time it took the contractor to complete the test).
Hope that helps.
By the way, just saw h4labs - you should check out duolingo app if you haven't, it's pretty amazing.