I insult them back. "Complement" the way their fat hangs over the sides of their jeans. Tell them how i wish my wife had cankles like those.
Ive never seen a problem with getting on someone else's level. It makes the fight so much more fair. And i did this all through highschool. ((Almost 21

and i still act like a kid!))
Sure, its not the best thing you could do, but fair is fair. "Treat others the way you want to be treated" doesnt work in this day and age. If war breaks out, you dont carry cakes onto the battlefield hoping they will bring you cookies in return.
Besides, people who are a dick deserve to be put in their place.
I mean, my wife says im the nicest person on the planet. Its rare for her to see anything but glasses, and a smile on my face. But if people are mean, stupid, or just plain dicks, return the favor. Fair is Fair.