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!
F31 330d M Sport (Alpine White / Red)
Packages - Parking, Security, Safety, Visibility, Pro Nav, M brakes, Adaptive S, Sport steering, Comfort Access, Sun glass, Mirrors, Heated seats, Surround-view, HUD, HK sound, Speed display. Mods - 405 wheels, M performance aerodynamics. M perf brakes red.
My toy - E92 M3 - (M.White + red)