seriously, though...I would keep my eyes open and see what is "traditional" in your building. where I live, the valets have signs prominently placed that say, "TIPS APPRECIATED". well, alrighty then. a lot of people tip every time their car is retrieved, others seem to tip generously on an annual basis in the obligatory holiday card. personally, I don't see that one way or another gets better service. honestly, it seems the people who complain the loudest are the ones who get the most prompt attention.
me, I go for the panty (or not) flash. THAT seems to work every time!!