My referral program consists of: A thank you note for the recent referral with a $10 gift card to Home Depot, Chili's, or Blockbuster Video. The more referrals the customer provides the better the gift cards get.
I used to give the customer a free Mowing but realized that most people would probably prefer something that there not already receiving.
$35 gift cert to a nice restaurant close to their house. I do it as a surprise.
I send out a letter each spring reminding people that the best thank you they can give me is to refer a friend in need of my services. If you don't ask, most people think you are too busy to take on additional clients.