I've been using eLance.com to source VAs for the last 5 years. Went through the full cycle (trying to do it as cheaply as possible and getting burned.. to now paying a reasonable rate for a US based VA who I can pick up the phone, speak with and understand well) Great success with one VA who I hired on a as a remote sales assistant after working with her for 1 year.
Tips:
1. If you are paying less than $10 an hour, only assign operational/procedural tasks. The incentives in this range are to demonstrate they can "work quickly" so that you keep assigning work. Quality suffers
2. Expect to pay $500 to $1000 and between 10 and 20 hours of your time "trying out" a VA.
3. Create a wiki with lists of commonly asked questions / answers / information. Have the VA update this with any question that you ask so that you are only asked questions once. This becomes enormously valuable!
4. Ask and give feedback reguarly (what can your VA be doing better, and what can you do better in your business) I've been really suprised by how much insight someone actually dealing with customers email every day has. I changed an entire product line because of this.
5. Do not, under any circumstance, have a VA do fulfilment for you. Pay a profesional fulfilment service to do it.