I do a maintenance contract at a lower rate, estimating the number of hours needed to do general maintenance work for that particular client (with an agreement to adjust, if necessary, after 3 months). Additional hours above the contract are billed at my best rate - I'm not in the insurance business, and I'm not going to assume the risk of my client's president opening an e-mail that infects every workstation with a worm, but I'll give them a fair price when "$h!7 happens".
The benefits to the client - I'll answer the phone if "my" network crashes at 2 a.m. on New Year's Eve, they know they'll be on my schedule every week, they have one person to gripe at if something goes wrong - basically, I take the responsibility and the headache of keeping their network humming. I'm not answering midnight calls for customers who say "yeah, we probably ought to do maintenance, but it costs too much." Also, my 2-hour minimum drops to 1 (less if I just drop in for an emergency and don't feel like being there); I have 24-hour access to their systems and often perform critical maintenance before they know there's a problem; and they have less overhead managing my billing (a fixed monthly contract doesn't require the level of checking that an hourly contract would, and I give them better rates because it reduces my paperwork). I also know their network inside and out.
What you cover - and how you spell it out - depends on you and the client. Some clients can only afford $X, and I tell them that I will prioritize work each month up to the contracted level, then fill them in on what else should be done. Some clients prefer that you spell out in a contract the exact tasks you'll perform. You should know what the client wants from the contract before you sell them a contract, anyway.
