I send a Thank You card expressing our appreciation for their referral and include a gift card for $10.00 to a restaurant, coffee shop or store I think they might enjoy. If its a customer we also offer them the opportunity to take $10.00 off their next service in lieu of the card. In these times many appreciate that even more.
I thank them sincerely in person and then follow it up with a handwritten card again thanking them and assuring them that I will reciprocate the favor at my first opportunity to do so. I include in the note that I've donated to a charity, in their honor, that I know is of interest to them. I do so because I like to donate a portion of all my commission to charity and this is another way to do so while also saying thank you to the referral source. The charity can be anything from a sports team, non-profit, church, school, etc. So it is a nice way to give to the community also.
We always write a handwritten thank you card, even after an initial sales meeting, and then if we make the sale, we send a gift basket, or try and take them out for a nice biz lunch if they have the time.