I put together a video with some tips. The most important thing you have to remember is that the primary role of your service agreement is to define the relationship between your company and the client. The second most important role it plays is to define the financial relationship between your company and the client.
Sometimes it's tempting to list out all the details of your specific offering. But that's not really necessary. You certainly don't have to list all the desktops, laptops, and printers. that's obviously too much detail.
The service agreement should be a broad outline that addresses the relationship, not the specific plan that you're providing this year. Those details can be on a referenced sales sheet or web site. You might even stample them to the back of the agreement as an amendment.
Remember: If your relationship is going great, no on will read the service agreement. And if it's going bad, no service agreement is going to save it. The service agreement provides very specific legal and financial guidelines for the relationship. The juicy details of your actual relationship are elsewhere.