Sending a friendly email to users just before their trial period ends is a great way to keep their attention and keep them from forgetting that they have a free trial. The best time to send the email to remind people to upgrade to the paid version of your product may differ from user to user, so try it out. You could, for instance, experiment with sending the alert at the 75% mark as opposed to the 90% mark of the free trial. (Please also send an email notifying them that their free trial has expired.)