Customers make the best sales reps. People who have used your product or service and are willing to risk their reputation to recommend your business. However most businesses do not benefit from this simple fact. The best way to is to simply ask for a referral. Given the prevalence of social media such as facebook, twitter, linkedin etc sharing information is quick and simple.
Another method could be to set up a referral program which offers your customer a discount off their next order or a monetary reward. For example, a number of recruitment agencies offer a ?250 voucher for referral people who get successfully placed and stay for a minimum period.
The benefit over traditional advertising is that you only pay when you get a customer and a referral is the gold standard in advertising a product or service.