1. Rewards programs. Give prizes/discounts/points for every referral. Points are nice since you can encourage multiple references and allow people to redeem points for free or discounted services with you, building repeat customers.
2. Useful info. Provide a newsletter filled with fun, funny, useful information (NOT corporate bumpf or sales pitches) and encourage customers to share it with friends and colleagues. Of course, you will have a single, effective sales/discount/introductory pitch in the newsletter.
3. Discount coupons. Give each customer three coupons, with their name/ID# on them (so they can't redeem the coupons themselves). The coupons will give new customers a discount on your services, so the existing customer is doing a nice favour for their colleagues or clients by giving these out.
4. Ask. You'd be amazed how willing happy clients are to refer you to other people. It should be part of your standard closing with happy clients to request that they let others know about their good experience.
5. Make it easy. Give clients the URL of that discussion board about your industry where people post about their experiences. Provide postage-paid postcards they can sign and drop in the mail to friends. Send them a well-designed e-mail outlining their savings/value that they can simply forward to people they think might be interested.
There. That's five. Do I get a cookie? How about a referral? <Grin>.