It's what your public thinks. You may think that a free hot dog with every inner tube you buy is the greatest idea since sliced bread, but if your public doesn't care for hot dogs it's not going to work.
You are not necessarily the same demographic as your buyers. It's important to find out what they like, and what motivates them to buy your product or service. Simple client surveys can give you a wealth of information about why your customers chose you. Showing off your new promo piece to a few of your favorite customers prior to going to print can also give you some interesting feedback.