Hi Kitjaii,
Several of the more popular outsourcing services provide a time tracking program that can help you monitor your project's progress. As you already know, oDesk provides such a program. However, should you want to work with a designer outside of a service, take a look at some of the freeware/shareware time tracking programs available, and see if you can get your designer to work with one as part of your contract.
Personally, I prefer to pay per project (not per hour) since a slow worker can unnecessarily drive up costs.
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