I used to just throw those tickets away because they were not enforceable in the past. I haven't got one in several years though, and I'm not sure of the legal status of these things now.
Edit: regarding what Flotsam mentioned about the voluntary nature of the program....
The Forest Service put the plan in place on a "voluntary" trial basis. They tried to say anybody that paid was voting in favor of the program, yet they also tried to enforce payment from people. The Courts ruled that if payment was a "yes vote" then people had a right to vote no by refusing to pay. The Forest Service kept putting these fake tickets on people's cars anyway, but stopped trying to enforce payment via the courts. Therefore I always just threw them away. I'm not sure if the program has evolved from there and maybe has teeth now...?
Before someone comments about how I should just pay, my response is that I already pay. As soon as the Forest Service starts actually making money on their timber contracts instead of subsidizing the timber companies -- I'll start paying for parking. (Assuming they still have a budget deficit.)