You can get 500 airberlin topbonus miles for becoming Heinemann & Me-member (German duty free chain) by December 31, 2013, and having airberlin as your partner airline.
This doesn’t require any purchases to be made before you get the miles. Signing up for the service is enough.
You can access Heinemann & Me’s airberlin page here.
Airberlin’s topbonus has many promotions for extra miles with number of partners. It shouldn’t be too difficult to get enough miles for an award ticket by taking advantage of these offers