BMW Owner's Guide to Winter Car Maintenance
The most basic thing you can do is make sure you make all your regular checks on time and that you car has the correct oil level, the coolant has the proper concentration, check the lights and the battery.
You might be surprised of the last one but your really shouldn't. During winter time, oil (which is still a liquid) gets thicker and harder to move around inside the engine block. Therefore, the car will need more power to start up around this time of year and the battery will be taking most of that effort straight on.
An adjacent problem is the alternator. Since the battery loses more power, the alternator will be working overtime this time around and might throw fits at you. Not only that but you'll also be using the seat warmers, steering wheel warmers, windshield and rear window defrosters, all of which take up more current than usually, straining the alternator even more.
After checking the alternator, it would also be a good idea to check the windshield wipers, as snow and rain will be more and more often around this time and good visibility is paramount.
Another nasty problem you could encounter would be freezing of the doors, windows and locks altogether. To make sure this doesn happen, apply some glycerin to the locks and around the seals and hinges, to prevent freezing.
If you're going on a long trip, pack a survival kit that has food and water in it and make sure you have a shovel at hand and a full tank of gas. In case you get stuck somewhere, they could turn into life savers.
As BMWs have rear-wheel-drive transmission (excepting xDrive models) you should consider winter tires. The DSC will try to keep you on track even with summer tires but you'd put your life and the others' lives in danger every time you go on the road if you do not switch to proper rubbers. The same goes for xDrive cars too. The AWD system won't work miracles.
Last but not least, check the heating/climate control. Usually people check their systems during summers when the AC starts acting up but the same must be done before winter sets in, to make sure you won't freeze to death (highly unlikely though).
This should cover all the basics. However, every car has its specific areas to look over. Older cars should be washed as often as possible as the grit laid down to de-ice the roads contains salt and sand that could damage your car's metalwork that is already weakened.
Old diesel engines might take longer to start and to help yourself out, you might want to use special fuel with additives. We even heard rumors of people getting a gallon of petrol when filling up their cars to help out during the cold season. We wouldn't recommend it though.