Guys,
Spo123 made a good list, but I think driving the car regularly and pretty hard in cold weather also helps. My daughter has our 1982 240D in Troy, NY now. This car would not start a few years ago in the summer! It now has a good battery, good plugs, a great starter (Fastlane), new engine mounts (broken mounts will waste starter engergy throwing the engine around under the hood, and not spin the crank), Delvac 1, Power Service and Redline, reasonably fresh filters and it has started flawlessly every day in the last two week cold spell. Troy has lots of hills and so the car gets a pretty good workout every day. I think it got down to minus 15 degrees one night. No block heater and no garage, so, while it needs a little persistence some mornings which undoubtedly strains the starter (kind of half runs for a few seconds, maybe ten, on its way to starting) it has been working well. No midnight starts unless there is a food run required. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)