Miles can be protected from expiry via status membership or with a Miles & More Credit Card.
Members who achieve Frequent Traveller, Senator or HON Circle Member status will retain their miles for the duration of their status. If they lose their status, their miles will once again be subject to expiry. Irrespective of whether they were earned before or during that status membership, miles are valid from that point on for a further 36 months and then expire at the end of the quarter.
Some countries offer the option of preventing mileage expiry with a Miles & More Credit Card under certain conditions.