Never let anyone tell you that word of mouth is hard to get. Word of mouth marketing is about listening and earning a recommendation. (Listening comes first!)
You can do both with a 5×7 card.
Copy this simple card included with every order from PrintingForLess.com. It combines a simple 2-question survey (with Net Promote Score) and a 25% off coupon for a friend.










One gripe with this, 0-10 may be too granular for some people trying to decide “Were they an 8… or a 7?” I wonder how it worked for PrintingForLess, and think a 5-star (or similar) system is more practical. Those who are bound to detail will tell you if you fell between two integers, anyway.