If a users membership is due to expire, a notification email is sent warning the user that their account may become inactive. This notification is sent automatically by the system and you can customise how many days before account expiration the email should be sent. When the account expires, a second notification email is sent warning the customer that their membership has expired.
Both emails include clear instructions as to how to go about renewing membership. Account/membership renewal is handled by the 'User Account Manager' interface and as when purchasing, clear invoices are displayed giving accurate details as to what is being charged and when the membership will expire again.