Recurring revenue is a very good reason to offer your clients maintenance service, and Brad Touesnard walks you through how you might be able to go about doing this. Among other good pieces of advice, Brad points out you can bake maintenance right in:
I love the idea of baking a year of maintenance and software licenses right into the initial proposal/contract, not even giving the client a hint that there’s an option to opt-out.
Of course, then you have to actually sell this stuff, and follow through. It can be good recurring revenue, but you have to be ready to work.