Become familiar with how to match colors, blend objects, and select irregular regions.
Also, to do a good photoshop, you need good, high-res images of cars to pull source parts form. Mostly I use DieselStation. Google Advanced Image Search is also helpful, but remeber to set size to "large."
I found GTJugend's link more helpful cause it has pictures while the other is in forums which is some what of a downer. Thanks for the link I'ma be pracin on a camaro.