I bought 4 cars in the past 20 months and I have always gone to the dealer with a broadspeed print out. I've never yet paid more than Broadspeed and in usually less.
In my experience in order to get the best price:
Be ready to buy, and be serious - let the dealer know "I will sign today if..."
Order the car, and sign, pay a deposit, then add options. Insist on the same discount level (e.g. 10%) on any future upgrades. Do this asap though.
Good luck... and avoid the Audi garages... you know you want a BMW!
F10 M5 Competition White / Sakhir Orange
Loaded, Akra carbon titanium exhaust, Carbon trim, Carbon spoiler, RKP carbon front lip + diffusor.
F31 330d M Sport (Alpine White / Red)
Loaded, 405M 20", M perf: skirts, brakes red.