Just take time experimenting in Photoshop.
Where do you place the camera? Is it in a 'realistic' spot? That's a good start.
Could you make that picture in real life? That helps when trying to make it realistic.
I like the first 4 pics of the DTM A4, because they could have been made during
a real event (squeeze your eyes a bit lol)
But that took quite some PS'ing, they really look nothing like the pics that I originally exported from my PS3.
What really helps though is reading car magazines and studying their pictures