How can I keep from feeling negative toward customers?

I've been a retail salesman for the better part of the last 10 years and something I've noticed is that customers are much more educated, and smarter consumers...that is, they're not impulsive and buy right away without asking legitimate questions. Believe me, that makes my job MUCH easier (answering questions now saves you from having to answer questions later). However, something else I've noticed is that customers are increasingly rude, ill-tempered, and mean-natured. Now, make no mistake that I'm a pretty thick-skinned individual. Having worked in retail for a while, it comes with the territory. But I'm finding that I'm beginning to show "cracks" in my "skin." Customers will come in and ask loaded questions and do it as rudely as possible, they'll insult you, your business, or just act ignorant or disregard anything you say just to get under your skin. I'm finding it increasingly difficult to zip the lip and just let it go but customers get more and more blunt, rude, and just flat out nasty. I know I probably sound like a cry baby here LOL but I'm a very good salesman and having these feelings toward customers shows whether I want it to or not. Heck, I haven't even said anything or lashed out yet. I've heard that there are actually people who spend their weekends going from store to store "abusing" salesman by asking loaded questions or putting them in awkward situations to throw them off their game and not appear as confident as they're used to. How can I deal questions and comments that are meant to put me on the spot or make me feel uncomfortable as a salesman? How can I deal with customers that come in and make it clear right out of the gate that they're going to be as difficult as possible? I'll let you know that the vast majority of customers are genuine, good natured people looking for a good deal. But how on earth do I deal with people who just want to come in and make you squirm? HELP!!!