With the Refer a Friend Program, offer not just a Dollar value incentive, but a percentage coupon discount, like Who Gives a Crap uses.
Or a free entry into a prize draw.
For example, my business runs a contest where we give away 3 Free Boxes every year to a lucky subscriber. We wanted our referral program to offer additional entries into this draw, rather than a money discount. So subscribers who refer a friend get an additional entry, and so do their friends.