This is a very general question. Some considerations are:
1)research rates before hiring. If the person is charging a lot more or less than the median, ask why and get a thorough answer. You may turn up a response that means the person is not suitable for your project or misunderstands the role.
That being said, clarify the rate structure (such as simply by the hour, day, project vs. time and materials), make sure you are not treating the person like a salaried employee (tax issues), do a simple contract with clear points, and address how to deal with scope creep, unintended time extensions, and time off.
2)treat the person fairly and pay quickly. I stayed with a really irritating client because even with his difficulties he always paid immediately and never questioned my hours. That being said, I always submit timesheets with my invoices.