NEVER GIVE THE VEHICLE BACK.....that's just horrible advice, and you'll get shafted so hard that not doing anything and simply paying for the vehicle you can't afford would be a better decision in the end.

The best option is selling the vehicle off privately, then paying the full loan off ASAP....sell whatever you have to for this to happen. If you default when someone else owns it, you can expect not just civil issues, but possibly criminal issues as well....so pay it off quickly.

(edit): by the way, the only wsay to get upside down on a car purchase is to have the loan extended to reduce monthly payments (depreciateion exceeding rate of payment). If you cannot afford the car under a normal duration loan, don't buy it. I know it may sound harsh, but this is one of those rules that if followed, you're pretty much ready when the crap hits the fan.