I've also seen clear shelf paper used.. cheap and easy temp idea.
Also, I've heard people like this stuff:
http://www.buyroadwrap.com/
Pretty cheap to use for a big track day or for a long cross country haul. Sort of a temp version of my film. It's thinner so it doesn't give great protection from big rocks, but it's great for "sand" and other smaller track debris.
My paint is in great shape four years after putting the 3M Venture film on the car, fill still looks great but has a sandblasted effect in some spots, of course without the film my paint would have been blasted instead. And still no yellowing or "issues".