>>My Google search has turned up lots of hits, but none that stated what I need to state.
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>>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.
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>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.

Thanks Thomas, yes we are arranging that when the hours worked reaches less than 10 hours a new retainer will be paid. This should be fine for the amount of work anticipated but we will adjust if necessary.