A really good way is to make sure you are using a system where you are only paying for new sales. For example, you can do something like: "Get $10 back if you

A really good way is to make sure you are using a system where you are only paying for new sales. For example, you can do something like: "Get $10 back if your friend makes a purchase using this $10 coupon." That way you aren't paying out until it results in a new customer.--much less risky than other forms of marketing where you are paying before seeing the results.