If nothing else, you can review their closed file once each year and see if any follow-up work, contact, or inquiry might be appropriate. At minimum, you can call to verify their current status, get their e-mail address, and give them yours. Make sure your accountant knows about the types of work you handle and that you'd appreciate a referral should a matter arise with someone they know. Many accountants are masterful marketers and will instantly understand your message. But do not make the mistake of assuming they will refer you business without your asking them to.
People like to be courted.
Tell them that, while you're pretty busy, you'd appreciate their sending a good client to you when and if the situation arises. Most accountants know many lawyers, but you may be one of the few who specifically reminded them that you'd appreciate an appropriate referral. You have one or more agents for life, health, homeowners, and car insurance. Their whole game is marketing. Maybe you should take each insurance agent you deal with to lunch once each year and find out about his or her work, new products, customers, problems, etc.