[judofyr]

What about sending an email like this a few days before the trial expires:

    Hey <name>, You signed up for our trial, but you've
    almost never used it. Click HERE to extend your trial
    for another month if you feel you need some more time.

[mikeleeorg]

I like this idea. You could also check to see if the user has used your product frequently within the trial period or not. An infrequent user could be sent such an email to give him/her more time.

This email also acts as a nice reminder for users who may have forgotten about your product. I've signed up for so many betas that I sometimes forget.