>If you don't feel comfortable with their ability or willingness to pay in a TIMELY manner, or if you can't function w/o the expected revenue, I'd ask for a fair percentage up front and/or make and agree upon a deliverables schedule where they pay based on milestones.
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>Never get stuck in a contract you think might not pay what and when you expect (unless you are absolutely desperate and you have NO alternative).

I did some work few months ago. We had an agreement expressed in few letters that I was going to be paid certain small amount for the work. But after some time the guy has disappeared and I just didn't try to re-establish the contact...