Our Painting Service Contract!

I have a painting service contract for our customers. It has worked out well for the last year but we haven't signed up a lot of customers. I think we have 15 customers on service contracts right now. I really want to vamp this up but I am hoping to get insight from other businesses on best practices for service contracts. I'm also trying to figure out if I should just sell as many as I can and then hire a full time person to service those clients, or keep slowly adding them until I can hire someone full-time to do it. I have been hesitant to go after this super hard because I don't want to be in a position where I can't get to our service contract clients for too long.