I send out and/or drop off cards to my "good" customers at least once or twice a year.
They are simple business cards with my standard business card on the front and on the back a 20% discount, parts and labor, printed on the back.
I take the time to write the customer's name I am sending them to on the card. And the letter I include with the card tells them that if they like my service to please give out the card to folks who might need my services.
And for every card that is turned in, they will get $20 bucks.
I usually spend around 300-400 bucks a year redeeming those 20 bucks a pop cards.
And they usually generate me 80-90 bucks gross profit after the 20 buck payout and 20% off per card.