Start by going to the car manufacturer's website and click on that little "Build It Now" button. Figure out which options make sense for you, then go to the dealership and check those options out firsthand -- you probably won't find a car with all of them, but you can try the seats of one and the stereo in another. When you've made up your mind, skip the salesmen entirely and sit down with (or even email) the sales manager with your order and how much you're willing to pay.
Custom ordering is also cheaper, if you're smart about it
Let's say you want the New Hotness Coupe 27B, and the ones on the lot have navigation and the high-end stereo, but no moonroof. And you really, really want that moonroof, way more than you want the fancy-ass stereo. When you place your order for exactly what you want and nothing more, you'd be surprised how much just a little restraint can wind up saving you -- it's often several thousand dollars.