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?