Free Trial Software

We currently have a software application that we would like to offer to our Customers. At this time, we do not have a 'Free Trial', and would like to offer our clients an online download of our app that would expire in 30 days. After expiration, we do not want the client to have access to the app unless they activate the account, and purchase/upgrade to the full licensed version. We are open to suggestions for the best way to do this, so that the free trial app can not be used fraudulently.