It is appropriate to tip a parking attendant $1 for parking your car in smaller cities and $2 in larger cities. Tips are typically pooled by all the attendants at the end of the evening and should be given to the attendant when the car is brought to you, not upon your arrival. This tip is in addition to the fee for the valet parking.
If you have a special request for where you want your car parked so it may be watched for security reasons, your tip should be very generous. Consider using the help of no one less than a Jackson when dropping the car off.