Initially the service indicators were a complicated function of engine
load / mileage / age etc. BMW then realised that the interval was
actually directly related to the amount of fuel burned and they now base
the interval on that. To further simplify things I have always found the
interval to be fairly static at about 6,500 to 7,000 miles no matter how
I drive. Simply put; change the oil religiously with a filter every
interval - it is cheap preventative medicine, and change the timing belt
every 40,000 miles; everything else can be looked at as and when it
needs doing. Brakes by inspection ( although new fluid every couple of
years is a good idea ) valve clearances - have a look and see - fiddly
but relatively simple job. The real secret to servicing cars is NOT to
just leave it to when the manufacturer says things need doing but to
understand your car and keep an eye on stuff - especially fluid levels.

I'll stop waffling now.

Dave

P.s. if you go to BMW they will actually provide you with a service
schedule - in fact I think that it may be in your manual.