Have you considered a referrals type scheme?

I mean offer your current customers free maintenance or some other carrot for referring signed up clients to you.

For the new clients thy bring in offer them a 10-15% discount, or maybe free maintenance extensions?

Also for any new clients not from referrals offer them the same as above

I really believe a referral scheme works fantastically, I mean your clients are your best advocates! They love your work and want others to enjoy the same things as them