My clients get their first months maintenance free, anything over an above that covered by my maintenance clause.

I also have a client referral scheme written into my contract:

40. Client Referral Commission Program
The Developer recognizes 'word-of-mouth' advertising as our most favourable method of developing new business. As such, we want to reward customers who are pleased with our work and refer us to another individual, business, or organization.
If you refer our services to another party and we ultimately establish a contract with that party, we will provide you, the Client, with two months of free maintenance service. For Clients who regularly help us attract new clients, this can result in a virtually free Maintenance Agreement.