A: You give them something to show their friends.
Great example: I bought a pair of amazing pants from Cordarounds. Everyone started asking where I got them.
Guess what? In the box were a dozen little cards, each featuring a unique product photo and their web address. That gets a dozen face-to-face referrals, 20 cents each. (Get your own cards here.)
Lesson: Make it easy for your fans to talk about you.