It's sometimes said that the hardest thing in business is to get a customer to part with their first pound. (Or dollar, or euro, or rupee or whatever).
It's much easier to get repeat business off existing customers - because, unless you've screwed up royally - they already know and trust you.
But are you taking advantage of that? What systems do you have in place to make sure you go back to your existing customers and see if there's anything more that they need? It can be as simple as sending them a postcard a year after the job was completed - just to keep you in their minds. Or are you content to just let that potential cash slip through your fingers?