Airberlin is running a promotion with Sixt rental offering the opportunity to earn more than the normal 500 bonus miles with every rental. Once you reach 5 rentals you will start earning 1,000 miles per rental. Hit 10 and you will earn 1,500 miles per rental. Make it all the way to 20 rentals and you will be earning 2,000 miles per rental. If you are frequent Sixt renter, this could payout. Rentals must be completed by May 31, 2015 to earn bonus miles under this promotion.
But you don’t actually have to rent from them to get something from this promotion. Just for registering you will receive 1,000 topbonus award miles credited to your account. You have to register your account by 7/31/14 in order to be eligible for the 1,000 bonus miles. Reports have been coming in showing miles posting w/in a week. The actual promotion page is in German but if you have Google browser it will translate for you!