You can also consider flying with AA and BA. AA's programme is probably the best for earning status with only economy flying.
Assuming the more direct long haul flight, you can take routings like AA or BA to LHR then BA to BER or AA to CDG then Air Berlin to BER. If you hit AA EXP status you can use SWUs on the long-haul AA segments.
How often are you going to be travelling?