Tip number one dont allow a chick on the top of ur hood! No matter how good looking she is.
You know, I wanna contribute an opinion on chicks-with-cars pics - not that I'm an expert or anything, but I've looked at a few
The thing I see the most that bugs me TO NO END is when the girls look uncomfortable posing. Cars are completely inanimate, but the human face is so remarkably expressive and instantly recognizable that the slightest amount of awkardness or discomfort immediately draws the eye away from all the good stuff in the pic (car, the girl's body or outfit, etc.) The brain takes a fraction of a second to sense emotion, and before you even get a chance to take in the whole pic, you get a gut feeling that the model is not happy. It kills what often could have been a good photo.
I feel like this comes out the most with non-pro models, who might feel awkward or shy, or with any girl who is either being asked to pose in an uncomfortable position, or is leaning on a hot car. They might even be smiling, but contorting their bodies or tensing themselves to hold that bad pose, which totally shows.
The best pics without a doubt are when the girl seems really comfortable with the camera and the car, like she's having fun doing the shoot. I don't know about you guys, but to me those differences are clear as day. I noticed this while browsing the babes-with-Buicks thread, there are plenty examples of both in there.