Converting Clients to Service Contracts

[computerdoctor]

I've had quite a few businesses call me to do work on a specific problem/problems. Most have called me again, but often much later. I'm not nearly as good at sales as with technology, so I wonder how do I get them to sign a service contract, especially smaller businesses. I have an instance now where the client is generating a large bill (1st call). When do I say, for $450/month, I'll do xyz but here's the bill for $1500 for this last week.