Driving a long distance in hot weather without AC?!

Serious hints? OK. Here goes.
You.
* Practice getting used to it. Do some aerobic stuff outside, and build up some tolerance. (I'm convinced I came in 1st in class autocrossing on a very hot day because I had been working on the roof every day for three weeks.)
* Start hydrating before this weekend. Some electrolytes and some potasium helps. Both of these are needed to make good use of the water your drinking. It doesn't have to be straight water, but careful on the other extreme as you might have guessed. Gatorade is too much electrolyte - add 2 parts water.
* Light clothing. My first drive to Fla in the 'cuda I bought TEVAs, and wore a button down shirt. Shirt can be unbottoned while driving but the LL Bean look heps make you look repectible. After all there's no breeze sitting on side of the road saying Yes sir, no sir. ;)
* Have some cardboard and crawling/or under the car/changing a flat clothes 'cause that light clothing aint no fun when your doin' roadside repairs.
* Attitude. Just think about driving and not about the situation. It's like a mosquito bite or a cut. You can't stop working. You dress it and move on.

The Car
Most has already been said. I once tried the wet cloth & ice in the vent idea. Wasn't very impressed. Towel or something on the blck vinyl seat - absolutely! Drive in the shade and cooler hours as much as possible. Yep - gotta love them vent windows and big door floor vents.

Some folks find bandanas a good addition to washclothes. They even sell various stay cool bandanas etc in outdoor and workgear stores.