Don't take to BMW SA

. In all seriousness, I found a good and reputable shop, established a good relationship with them and that's it. Don't have to be paying the bills for a fancy dealership which high labor costs.
Only other thing I can think of is don't buy notorious cars. Off the top of my head, the pre 2015/16 N63 and S63 engines. I am sure there is other engines on the list but I know mainly about these. That there will save you a ton of money lol.