Sending low res preview to the client

[Arden.]

I assume most of you guys use the process of accepting partial deposit before starting work and the rest is to be paid after delivering the final version, and same as me you send your client a preview of the final version and await for payment before sending the high resolution version. I'm wondering how to avoid being ripped off if a client vanishes after receiving the preview. How big is your 'low res' version? Do you use any other precaution when working with new clients?