What a refreshingly great question
I wish there was an easy answer! But I do hear your pain.
On our UK business directory, we decided to ditch the annual subscriptions model and offer one off payment lifetime listings. So now we have your problem, after we've sold our services to a customer, the chances are they won't have a need to talk to us again
So we need to make sure we keep up the volume of new clients. Now this will be down to marketing and advertising, and getting this tuned correctly worked for us.
After analyzing on and offline spend / versus results, it was obvious it was online. Our pages need to be appearing in googles results, and the beauty of online marketing is you can monitor and track everything.
Although sounds obvious now, it took a while to realise which advertising and marketing avenues weren't working or couldn't be tracked.
Also, especially after the last couple of years, we decided to diversify and now have different revenue streams, which builds in a safety net if one revenue stream dips.
Headsocial touched on a good point, maybe you could diversify slightly into maybe garden work, or maybe maintenance contracts for local schools/govt etc.
I don't know what actual services you offer, but I know our 1 tree in our front garden gets a hair cut once a year, and we always use the same chap, year in year out. I know he advertises himself as a tree surgeon, but I'd bet a lot of his work is residential tree pruning.
