While guests expect hotel maids to be honest and as discreet as the Secret Service, many travelers don't bother to tip for a housekeeper's excellent service because they don't know how. "Sometimes they slip under the radar, but they are incredibly deserving," says Dan Post Senning of the Emily Post Institute in Burlington, Vt. "They deserve a tip of $1 to $3 a day."

Because the maid who cleans your room might be different each day, leave a tip daily, says Nancy Moore of Best Western hotels. "Leave it on the pillow or on a table with a note. Be really clear."

Tipping can be confusing but most service providers appreciate a tip for excellent service.

Tipping is not obligatory, but it is part of etiquette. And etiquette, as Emily Post always said, is a combination of manners and principles.

"Manners change with times, but the principles behind them — honesty and respect — do not," Senning says.