1) Make sure you get people with English of 4.5+
2) The people who are actively responding to your posts are likely to be among the least talented people in the system, seek out people with high ratings and invite them to your project.
3) You have to recognize and play the numbers hiring on odesk, say 40% of the people are able to do what they say they can do. Hire 2-3 people, let them know they're in a runoff, after 2 weeks one will clearly be the most talented (or said another way will actually be able to do what they claim they can do), end the contract with the others.