Compensation for Service Contracts.

I have recently opened a retail computer sales and service facility in my town. I have customers that are interested in purchasing some sort of service contract. For instance, I have one lady that wants to be able to call and ask technical questions when she needs, as well as wants to be able to have her computers repaired when needed. What is the best way of doing this so neither party is cheated? Should I used block hours or a monthly charge? I normally charge $65/hr for onsite service, and was planning on dropping that to $40/hr for contracts. What about the phone calls? What should the minimum per month be?
Thanks!