>I have been doing contract work for over 10 years on an hourly basis. The client's generally pay quickly and it's worked out well for all.
>
>Now I have a new client who wanted a fixed price bid. This is the first of at least 2 projects I'll be doing for them, probably more in the future. This first contract is rather small, and the next one is bigger.
>
>The question is; should I ask for a retainer? If so, how much? If not, how do I ensure that the client won't renege and leave me with nothing to show for it?

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).