Ask HN: Freelancers: have you had success with client referral schemes?

The first thing I suggest doing is not call it a "scheme." Focus on building relationships, not running a scheme. When you build a relationship (trust and respect) then the referrals will come.

You may need to push a few people to make a referral ("Hey I have some availability next month, know someone like you who can use my help?"), and they'll do it without any scheme or reward.