Devote one hour each day to calling your existing customer base.
Start off the call as a customer service call and then if you hear that your customer is happy and enthusiastic then simply ask them directly for a referral. As a courtesy, follow up your telephone call with a nice handwritten thank you note.
It's important that you ask for a referral on the spot versus your prospect saying, "I'll send you a few referrals later." If this ocurrs, your chances of getting a referral are slim to none.
Keep track of how many referrals you get and how many referrals convert to actual sales.
Another suggestion that you could try is to send a direct mail piece or an email offering your customers a free gift in exchange for a few solid referrals.