What kind of product are you offering? The answer to your question differs based on your customers and potential customers. If you're in a more B2C format and selling to individuals, the process would differ than a traditional B2B.
A few basic ideas:
I would build a creative referral program that gives them something when they refer you to new or prospective clients. We have given a credit towards our customers software renewal, future discounts or simple things like a Starbucks gift card and a nice thank you note. If most of your customers are local, sponsor an event and ask them to bring their friends. Would love to hear more about your business and what you're currently doing to see if I could help.