Ahh, finally a question I can answer. I happen to be an amateur photographer as well as a BMW fan.
Ok, so first things first. Chances are you're taking the pictures with a digital camera. Unless you have a high quality scanner, high quality film, and high quality printing resources that's probably the most cost effective and hassle free way to do things. Either way, these are some guidelines..
A) IF YOU'RE USING A FULLY AUTOMATIC CAMERA OR A REGULAR DIGITAL CAMERA The key to getting good pictures with this type of camera (the kind most people have) is to give the pictures the right exposure. Different exposures have different side effects, and you can't really control them with an automatic. So to get the best pictures you're going to want to monitor the conditions pretty closely. First, make sure your car is clean. That might sound trivial but if it's not the picture will look worse; as will your car. ***Second and most important, shoot in "open shadow." To understand the idea of an open shadow look at a large tree. When the sun is part noon, the tree will cast a shadow that is darkest at the base and lightest at the farthest edge. Open shadow is when you get to the edge but are still in the shadow. If you take all of your pictures in this part of the shadow they will look good. Note: See if you can get the background to be shadow too, or the contrast will look bad.
B) IF YOU'RE USING A MANUAL/SEMI-AUTOMATIC DIGITAL OR NOT: Do what I said above about getting into some open shadow, that's the most impoartnt thing. If you can set your shutter speed, it should be around 1/125-1/90 sec. The aparture will obviously vary accordingly.
****Don't shoot in deep shadow. Don't shoot in direct sunlight. Don't shoot with half the car in the shadow and half the car in the sun. Avoid using the flash at all costs****
That's probably way more detail than you will ever need, but I still don't have a BMW so I'm one of those kids that hangs out at these boards wishing I had one. Gotta prove my self worth somehow...
pictures turn out real nice during a cloudy day as the clouds acts as a diffuser. Direct sunlight wont give the best photos; for that you need a diffuser.
best quality photos will result by using SLR or digintal SLR camera. though they are mighty expensive.
also composure. think outside the box, think what would look cool and experiment. macro close ups, or wide angles...try using the rule of thirds (imagine drawing a grid on the viewfinder in 3rds. 3 vertical lines, 3 horizontal lines. with that in mind, have the subject(your car) on one of the intersecting points. basically, this avoids having everything centered --which is overly done)