If possible, start with small tasks, and even things you could do yourself. That way you can truly judge if someone acts professionally and does a good job.
I have found some really good professional advisors through referrals, as that means both sides have more at stake (their reputation).
Also, don't stint on paying folks. Pay them before the due date if at all possible.
If using a freelancer who is good and dependable, nothing will surprise them more and move your work to the front of the line than an unexpected raise in rates. (Do this after 6 months or so of good work.)
It's obviously not surefire but with decent screening, it's been cost saving to spend more money on freelancers of all kinds.