slight OT...

Be knowledgable about the product. When I go to pick something up from anywhere, if the salesman obviously doesn't know about the item then I know he's just selling me something to move me out of the store and move another customer in. If I come in looking to fulfill certain needs and the salesman can't help, then screw that salesman, I wanna get done what I want, not what he wants.
When people ask for mod advice here, I always ask "what do you want to do with the car? drag it, auto x it, show it, etc." When I worked at Radioshack for commission I would find out their wants and then, cause I damn near memorized the catalogue, I could make intelligent suggestions.
After you establish what the minimum they want is, then you can start the "super-size it?" type suggestions.
Lastly, I know this sounds sexist, but if you can try to sell to women, its WAY easier. Its predatory but hey that DVD package in the Explorer would be great for the kids, ya know? And how about the navigation, don't you hate it when you're lost and are late? If a man and a woman come in together, cater to the woman. All husbands are whipped. Thats all I got for now,

good luck man!