The most innovative ways, might also actually be the least innovative ways:
* Send them hand written thank you notes for their business.
* Interview them about their successes, and highlight them in your blog/on your site.
* Where appropriate, send them business from other customers
* With their permission, talk about them in your PR and interviews even more than you talk about your product
* Tell them whenever someone signed up because of their personal referral
* Actually answer the phone