I've usually tipped $1 or $2 to the person who takes it and the person who brings it back. If the weather is bad (rain, really hot, etc.), I might add another $1 or two, especially if the valet has turned on the A/C for me (when it's hot).
$2 when they bring it back to me. $3 if its raining. I've seen several times where the person who fills out the claim card and takes the keys is not the person who parks the vehicle.