A tenants' association (Mieterverein or Mieterschutzverein) protects tenants from abusive landlords. When you are a member of a Mieterverein, you can get help from experienced lawyers. They can check your lease, help you with a bad landlord, get your rent reduced, and help you sublet an apartment.

Being a member is not expensive: the Berliner Mieterverein charges 9€ per month1. The Berliner Mieterverein and Mieterschutzbund Berlin are the two biggest associations in Berlin.