I'm doing the same thing, so I've been searching the forums for answers. Here's what I've found:
Disconnect the C02 and Liquid lines. Don't purge the C02 from the headspace. Keep the keg upright if you can, to prevent any leakage. If you can't keep it upright in the car, turn it upside down tonight and check for leaks first, before you put it in your car (the keg will travel fine in any position as long as there are no leaks. Keep it cold if possible, but if not, start cooling it off asap once you get to your destination.
I'm guessing you can sit it in a cooler with an open top, with some ice in the bottom, or a trash can.