Scour online communities for complaints about features that your competitors are missing, then implement them and notify the original poster.
Dinesh Agarwal of RecurPost ($4,000/mo) did this and saw between a 20% and 50% increase in referrals, depending on the channel. He was already active in many online communities, so he kept an eye out for these complaints and, once he identified and implemented the feature, he told the original poster that RecurPost supported it. Afterward, he kept an eye out for anyone else making the same complaint. Social channels like Facebook groups, Twitter, reddit, etc. performed the best, at right around a 50% increase in referrals.