I always give a thank you note with a studio gift card to my clients who have ordered prints or wall art. The gift card, of course, is an incentive to come back again or share with a friend. The thank you note contains something special about our photo session-- either a funny moment or a comment about how well things turned out. 

For clients who have business photos and headshots taken, I started thanking them publicly. For example, I may post tips about photos with their headshot on LinkedIn and thank them for the opportunity to have photographed them - I tag them and usually this gets their page extra visibility and it gives their feed a boost. 

I always ask happy clients in person, after a session, if they will refer me to three friends. Being specific with how many friends seems to work well.