>My Google search has turned up lots of hits, but none that stated what I need to state.
>
>This contract includes being on call for support and maintenance and I have already agreed with the client that this is the best way to go, at least initially. I just need to find a way to word it properly.

Just make certain that there is enough time between advance being worked off and support calls coming in. That would probably include reporting H per day on each day including current amount of retainer left - nothing hard, as any excel sheet can handle this and show current summed, calculated and grouped values on top, but if there is a steady flow of small tasks such requirements can balloon the time needed, as on each tiny incident there is the need for the "paper"work.