Don't valet park a car that will tempt joy riders (e.g. sports cars or high end luxury cars; even cars with manual transmissions may be used to learn by the valets).
Someone's been watching reruns of Ferris Bueller. Easy dude, take down the mileage if you are that paranoid.
As for the amount it depends on a few factors.
If it's a Friday night and I know when I get the car at 2:00 in the morning there will be 30 others in front of me, I'll gve the valet $20 to park in front. $20 is worth not waiting 30 minutes for my car at the end of the night. Upon pickup $2 usually, $5 if all I have is $5s.
In the summer I usually tip $5...anyone who has to run to get my car in 115 weather deserves it.