Service Calls And Profit Margin

I'm trying to analyze my past business practices to plan for the future and I am wondering what expiriences others have had dealing with service calls, and service plans. How difficult is it to sell a comprehensive service plan? I'm also wondering what types of perspective customers tend to go with a plan versus on demand?

A. Always include some level of service with a price with every proposal, that is seperately signed to get the client used to signing a support agreement. (even if the support agreement is for 30 days) refuse to provide any BREAK-FIX service beyond that date.