I just leave a tent card with 7 day Guarantee and drying tips/care on one side and referral program/thank-you on the other. (10% Back to client as a discount. Explain I would rather reward them than the YPs ect..) Just mentioning the referral program isn't enough.
A list of ten names of friends can put your customer on the spot. Just for 2 dollars off for each name.
The best kinds of marketing seem to use some form of reciprocation.