Recent couple years, I have been using both Elance and ODesk alot for my venture ideas. Even being a technical person for all my life, it took me some time to use it very effectively. At here, I want to share some of the lession learned with all of you.
1. A Properly Written Project Description
There is a whole world difference between you are describing your project in plain English and in technical language. Using proper technical terms to describe the project can help your freelancer to understand the difficulty and scope of the project. Believe or Not, it saves money as well. Just imagine if you are trying to build a house without proper architect specs, it will be very difficult for the contractor to give you a proper quote. They cannot get into your mind to know what you want.
2. Provide Wireframe Of Your Project
One picture says a thousand words, please take the time to use pencil and paper to draw down your app idea page by page. So the developer will know what you want to have on each page.
3. Synchronize Your Lingual
Try to spend some time with your potential freelancer and make sure you know what he means, when he says "Modal Window, Authentication, Authorization, etc". Also make sure he knows what do you mean by "Go here and there". Giving a name for each page on your wireframe will also reduce the misunderstand on communication. Give benefit of doubt to people. When he says "no", make sure you ask what he means.
4. Validate their work on regular bases.
Nowdays any IT related work is verifiable by milestones. Anyone who tells you that he cannot let you test the app until it is finished, that is not true. You can use TestFlights for your mobile application and Heroku for Web Applications.
5. Take Screenshots and make videos clips of what you want.
If there is anything specific you want with your app, either a specific effect or transitions, it will be great if you can take some screenshot or make a video of it from some reference app. Like if you like Vine's Signup process, don't just say please make the signup process like Vine. Instead, take screenshots of Vine's App and send it to the Freelancer.
I hope you find this information useful, leave me a note if you have any suggestion and question.