Good question. I toyed with this idea for a while but realized that it doesn't work (if you do it legally/ethically) because most businesses aren't sensitive to the types of incentives you can offer them. If they signed up for $500 in year 1, and they can refer someone and get 50% off in year 2, they don't care. Either they have the budget allocated for year 2 or they don't.
Of course, you can offer the incentive to the individual, not the company, but then you're on thin ice. Giving someone an iPad as a thank-you? Not something I'm comfortable with. Taking someone to an expensive dinner? That happens quite a bit, both as a thank-you for referrals, and in the sales world in general. That sort of stuff makes me feel uncomfortable, although I get that it's a continuum from 'regular business meal together' to '$300 per-person meal at fancy steakhouse'.
Of course, you can offer the incentive to the individual, not the company, but then you're on thin ice. Giving someone an iPad as a thank-you? Not something I'm comfortable with. Taking someone to an expensive dinner? That happens quite a bit, both as a thank-you for referrals, and in the sales world in general. That sort of stuff makes me feel uncomfortable, although I get that it's a continuum from 'regular business meal together' to '$300 per-person meal at fancy steakhouse'.