Don't just limit yourself to current customers.
You usually think of getting referrals from current satisfied customers, but don't limit yourself to that group. One of my good friends and competitors in the consulting field asks every person he pitches for a referral to someone who might be interested in his services. If the original prospect doesn't buy, at least my friend gets something of value for his time.
Excerpted from Creative Selling: Boost your B2B sales