if you are not asking your existing customer for referals you are missing great leads that will result in sales.
there are many different ways to go about it but the long and short of it is, you need to make it a habit to ask your customers "who else do you know that would benefit from our products and services" and guess what...they'll refer you business.
even better yet...after you ask...ask them to make an introduction or a courtesy call or email to introduce you so that you aren't calling them cold.