Tips for writing a good service contract

So to be honest, I've never written a service contract before . We've thought about it, we've discussed it, but we've never done it. There is some debate as to whether this should come from the supply side, or from the maintenance side. Nevertheless, I am hoping to find some suggestions and tips from some of you concerning how to write what you feel is a good service contract with a contractor. How do I provide responsibility? Accountability? Your thoughts would be appreciated!