I don't use valet parking much, but absent some guidance, I'll tip $3-$5 on pickup. Where we live there's little valet parking and $5 is about par for paid parking for 1-2 hours at a restaurant or parking garage. Probably too cheap, but that's how I roll.
A further question- In parking, what's Good Service? As long as the car comes back in one piece and the radio isn't blasting Rap.