Hotel Tipping Etiquette

Courtesy of the NYT: Tip the housekeepers on your next hotel stay!

Perhaps because they were unseen, working in the room when the guest was gone, hotel maids were generally overlooked by many travelers until fairly recently. A seasoned business traveler would always make sure to have a bunch of singles on hand to take care of the parking valet, the bellhop, the doorman and the cabby. But the maid? ...[the physicality of the housekeeping job has] become more intense in recent years, especially as hotels in the upscale and luxury niches have added luxurious new beds and bedding. Take a look at those swanky beds. They have duvets. They have half a dozen pillows and heavy 300-thread-count sheets-- usually three sheets, rather than two, per bed. Now think about the extra work involved in making those swell beds. Yet in most hotels, the maids are still expected to do the same number of rooms a shift, usually 14 to 16.

Thus the importance of tipping. But-- in your experience-- what is the appropriate amount to tip?