Don't take photos in the shade. You might think its best for lighting, but in fact its quite the opposite. The more light, the better, however be careful shooting in the middle of the day, as the sun likes to cause "hot spot" reflections on the car, which become distracting. Try to avoid other distracting elements in the shot, like light poles, reflections, etc. Also, never shoot with a light/power pole directly behind the car, unless you're able to photoshop them out. Also, be careful of parking spot lines, they are distracting as well. Don't shoot on the shaded side of your car. Its not only bad lighting, but also very distracting. Don't be afraid to keep your car running during shooting, as you'll likely be moving it a lot during a shoot.
For some shots, I recommend picking up a good Circular Polarizing filter. It helps with reflections and makes colors pop. Also, don't forget about the rule of thirds, it really makes the composition better.
Taking pictures and posting them straight from the camera online is ok if you are just showing the object, but if you want to make the most out of the photo, post-processing is a must. It's not easy getting a perfect shot straight from the camera. Don't be afraid to mess with photoshop, contrasts, levels, saturation, sharpness, etc. Just go out, take pics, and mess around with them in photoshop for awhile.
I've done quite a bit of automotive photography, but I also have many friends that does it professionally. And while they won't give you all their secrets, most of them will point you in the right direction so you can find your own style of shooting. Hope this helps.