The reason a customer refers another customer to you is because they truly believe their friend or family member will benefit from your product or service.
A thank you note or gift as an incentive is a great way to show appreciation for the referral but it will not be the reason someone refers business to you.
Excellent service and true value are the most influencing factors in getting referrals.
Using an incentive program may make it easier to ask for a referral but you definitely do not need one to make a referral program work for you. You just need to be referable.