I find myself writing a retainer agreement for a client, and it just reinforces to me the notion that a lawyer must be extra careful about drafting the retainer agreement to protect them. Contracts such as these often involve people who aren't as sophisticated as you are, and courts tend to read them very narrowly against you. You don't want to get stuck with a bill for an expert (see here), or not be allowed to collect your fee at all (here). You really don't want to get into ethics trouble (here). So be careful. I suspect that talking about the specifics of a retainer agreement is beyond the scope of the blog, so I would advise you to contact me off-list or to talk to another attorney who can help you.
However, the one thing that's true regardless of what kind of contract we're talking about: write them carefully, read them carefully, and try to have a little foresight when doing both things to imagine what might go wrong.