In short, it is really about providing great customer service. I am lucky enough to work in a small town where word of mouth referrals are critical, and doing a good job and genuinely caring about your customers gets you noticed and gets people to stick to you loyally.
Plus I have a genuine love of learning about how other businesses operate, and get a kick out of improving how they can run using my software.
But as I get older, I find that my energy levels aren't suited to one on one development and constant support, hence my second all cloud based SaaS startup in my sig - really it is phase 2 while I scale back my personalised face to face customer support and explore a more distributed way of doing things on a global scale, rather than local.