IT Support Contracts

Hi, I was wondering if someone could help me.

we are a small IT company and are supporting our customers with support contracts, We used to do it by giving them so many hours per month for a fixed sum but now we are looking at doing this on a per pc, per server basis.  We know of a few companies that do this but manage to offer unlimited onsite support and remote support.  We are trying to gauge the write costs I just wondered how everybody else did it? do you include onsite support or charge when you have been onsite? does anybody have a sample IT support contract that they use for comparison, or some kind of template?

Thanks