My Experience coming from a Supercharged Acura and Modded 335i
Stick with the same Service Adviser all the time
Occasionally, provide them some non-monetary items such as meals. If I were to have my car serviced in the morning, I'll buy those boxofjoes from DunkinDonuts and 2 dozen donuts. If I have an appointment around lunchtime, 2 pizza's and a 2 liter of Pepsi will do you well.
My old Acura I had close to 10k+ worth of servicing done with all mods (boltons, suspension + Aftermarket Supercharger) and had my turbos replaced on my first 335xi. Only cost me 20-30 bucks for food.
2009 335i e92
2008 335xi e92