Wash the under side of the car as well as the top.
Regular maintenance on a German car is essential -- as my friend the German mechanic said, they are just like a Porcshe -- if you maintain them by the book, they last forever. Neglect the maintenance, and they will be junk in a couple years. All of us on the forum have experienced this at one time or another. It costs considrable less to do the maintenance corretly than is does to fix the trouble you get when you don't!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!