>>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.
>>
>>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...

Those kind of agreements usually work OK, but when they don't it can be a real PITA.

With small jobs, especially, it is hard to decide when to really set up an iron clad contract and possibly even have a lawyer (yecch) give his/her blessing.

Safest bet - get a solid contract in writing with detailed payment schedules and performance measurements.