My main criterion was good access to public transportation. I lived in various areas in Berlin already (each one has its individual flair), eventually I ended up in West Berlin close to Kurfürstendamm, which is a nice, quiet and central area.

The housing situation in Berlin is not nearly as bad as in other large cities BTW (contrary to what most people say), and although it got more difficult to find something nice for a reasonable price it's still possible.