All I can say is experiment using layers. I generally us white on a separate layer for highlights. A little burning and dodging can help if you adjust the pressure 10 to 30 percent. There is a lot to learn as you practice. It took me a few years to get them "Just right."
If it were easy there would be more people doing this. All I can say is check out tutorials....there are some on
Car design news
There is some awesome work in the tutorial section and portfolio section as well. Hope that helps.
- DV