If you're creating an incentive anyways, a great way to track this is with coupon codes.

Give everyone a unique referral code. The responses will be incentivized to enter it for their discount and you'll be able to see where it's coming from.

The type of cart or way you're collecting payment largely determines how you'll be able to do this. I used shopify which has some great plugins (like http://www.shopify.com/blog/6668676-new-free-shopify-app-bulk-discounts) but your own platform may vary.

Alternately, you can just add a form field and apply discounts manually. More time intensive but the tracking is the important thing. Once you can double down on what makes a good referral you'll be well on your way to creating a system that can repeat that process.

Feel free to give me a call if you have any more questions.