I currently have 12 freelancers under my payroll from all over the world (2 personal assistants, 6 transcribers, 3 graphic designers, 1 web engineer) and have worked with over 100 freelancers on an ad hoc basis. I mostly hire freelancers via global crowdsourcing sites like oDesk (currently Upwork), Elance or freelancer.com.

I am constantly asked "the secret" in hiring good freelancers. Especially since you're never going to meet them face to face. My answer? Communication skill.