As a frequent early adopter, I'd say the most important thing you can do after providing a useful product in order to retain early users is to actively provide excellent support. I'll tolerate buggy software longer if it is easy to report bugs (and I know that reported bugs actually get looked at).

Interacting directly with early users also "humanizes" your company, which makes people both more forgiving of early mistakes as well as more likely to put a little bit of extra effort into helping you get better.