I've found it very helpful to my business to follow up with my trial users at specific points in the evaluation process. To me trial users are one of my most valuable assets. Of course they turn into customers, which is extremely important but they also are often the users who find problems first since they're taking a critical look at the system. They also tend to ask very good questions and offer suggestions for improvements.

The trouble is that they're often very busy especially in my industry where most trial users are in IT. Along with that, most will need to bring my product to upper management before it can be purchased. Hence I've found that providing the following reminders at different points in the review cycle has helped.

5 day follow up
The first reminder is at 5 days. This is simply a 'hope the installation went OK' type reminder. This can help catch people who had trouble but didn't ask for help and also people who got too busy to get it installed and this quick little note reminds them to get moving.

15 day follow up
At 15 days a note is sent which assumes they've spent a little time in the system. The note focuses on pointing out some of the more advanced features of HelpSpot along with links on how to use those features.

42 day follow up
I leave the customer alone for the rest of the time until day 42. As I said many customers will need management approval before a purchase so I like to send them a ' your trial is running out' email a few days early so they have a chance to get the required permissions.

45 day follow up
This lets them know that the trial is up along with information on how to purchase as well as an invitation to get additional help if they need it.