We had a 2-sided referral campaign. When folks bought a Glowforge, we gave them a special URL and said, “For each person who buys a Glowforge with this $100 coupon code, we’ll give YOU $100 off.” It turns out $100 is pretty damn motivating. We saw users buy AdWords ads linked to their referral codes. We had users post to Craigslist and Reddit with their codes. The result was that many of our customers saved hundreds or thousands of dollars and we didn’t have to spend much on advertising.