I'm Jnovek's co-founder. We deal with businesses who are used to being trampled on, so impressing them is really not that hard--but here's a quick list of some of the things we've done:
1) We're honest and super-fast in responding to problems. This is huge, even if you say, "whoa, we have no idea what's wrong but we're working on it" that makes them feel like you care.
2) If anything does break, we're quick to give discounts. I firmly believe that if our product doesn't perform as advertised then our customer shouldn't have to pay for it.
3) I take any chance I can to "wow" them. For example, once an employee of one of our clients had to do a bunch of extra work to make our product work on their platform, while on the phone I had overheard her talking about how excited she was to go see New Kids on the Block in concert and so we had a NKTB shirt sent to her the next day. It cost us $50 but we have that client's love forever.
4) We send letters. Real letters. (In the mail.) When you send someone a hand-written note in the mail, they know you took the time to make that happen--little things add up to big things.
5) We ask our clients for suggestions, and work with them to make the product work better for them. It's very important that our client is making money with our product--not just paying us for it.
6) I try to understand how their business works and uses our product. For example, if I know they are usually busy on Wednesdays, I'll write that down and call them on another day.
When you sit down with a client, talk about their problems, and try to build products that solve those problems they get really excited--when you use their ideas, they feel like they have ownership of your product.
How does this pay off? We have clients who go to events and speak to others in their field about our product. And most of our clients are happy to receive calls from others about our business.
Our self-appointed evangelist clients are probably worth more to our company than the code we write.