The traditional way to tip for valet parking is at pickup only -- "upon competion of service."
15 years ago, I was tipping $2 at pickup only. But that seems inadequate in 2004. So now I generally tip $2 at dropoff, and $2-3 at pickup.
I know that the tip at dropoff is optional, but whole the idea of tipping is to take care of the service employees who take good care of you. Usually, at dropff, the valet parking attendent open doors, provides a friendly greeting, saves me the trouble of finding a parking space, and saves me time. So I really don't mind tipping $2 at that point.