Anything you can get that's moisture wicking and breathable will help loads on a longer drive. Hit up your local walmart and spend $8 on one of the cheap moisture wicking tshirts. Breathable/moisture wicking socks and nice, open shoes (anything with lots of open mesh, running shoes do the trick usually) goes a long way too, at least for me. Hanes even has a new product line of "X-Temp" stuff. Underwear, socks, undershirts/plain white T's. Spendy though. The underwear do help a bit. If you can make most of the drive after dark, that will help tremendously too. Keep both windows open. If it's not just blowing hot air, even just having your fan on can help.