Once you've been in business for more than a year, you should have a solid network of referrers. These can be clients, people in your network, other careers professionals, other business owners, etc.
If you don't have a network, get one! Think of some strategic partnerships you can develop. Be creative -- how about a referral relationship with your dentist? Or hairstylist? Or a divorce attorney? Or a mental health therapist who does career testing? Or a recruiter? An employment lawyer? The possibilities are endless.
Incorporate your request for referrals into your business. Tell clients that you get most of your new clients by referral -- and you'd appreciate, if they're satisfied with your services -- that they tell other individuals about you. Help them understand what kind of clients you're looking for ("Sam, as a senior executive yourself, you may comes across another executive who has been downsized and isn't having much success with traditional outplacement. If you are telking with someone like that, give me a call -- perhaps I can help him/her."
Develop (or enhance) your Reward for Referrals program. At a minimum, you should always send the referrer a handwritten thank you note.