Beyond split testing, it's a good idea to collect usage metrics for everything your users do on the site. At a bare minimum, you can send event data to something like Google Analytics and sift through it by hand to look for patterns.
Ideally though, you should be associating that data back to userids and bucketing it by whether or not that particular user converted to paid or let his trial expire. That way you can collect statistics on what things make your users happy so that you can know what sort of features to add in the future, and so that you can gently steer wayward users toward doing things that you know will tend to bump their chances of converting.
I've been writing a bit about this lately. Here's a better thought out explanation of the above:
In Google Analtyics we use the user id attribute and custom variables to track back their email address in reporting.
I always wanted to use kissmetrics, but the owner would never pay for it so we made due with our GA and custom rolled everything (which honestly cost more to maintain than the 200/mo kissmetrics).
Ideally though, you should be associating that data back to userids and bucketing it by whether or not that particular user converted to paid or let his trial expire. That way you can collect statistics on what things make your users happy so that you can know what sort of features to add in the future, and so that you can gently steer wayward users toward doing things that you know will tend to bump their chances of converting.
I've been writing a bit about this lately. Here's a better thought out explanation of the above:
http://www.expatsoftware.com/Articles/maximizing-saas-trial-...