The Hardworking Person You've Forgotten to Tip

American travelers are a pretty generous bunch. Virtually everyone tips restaurant staffers — 97%, according to a recent TripAdvisor survey. More than 80% of Americans tip taxi drivers, and 79% tip bellhops. Skipping the tip makes Americans anxious: 23% report feeling guilty when they don't tip, and one in three Americans has tipped someone even when the service was bad.

But when Americans travel, they sometimes forget to tip the people who clean up after them: hotel housekeepers. Americans are less likely to tip housekeepers than other service workers; more than 31% report that they don't tip hotel maids at all, according to TripAdvisor.

Now Marriott wants to offer a reminder. In a partnership with Maria Shriver's nonprofit advocacy group, A Woman's Nation, the hotel chain has launched a new initiative to place envelopes in hotel rooms where customers can leave "tips and notes of thanks."

"Hotel room attendants often go unnoticed, as they silently care for the millions of travelers who are on the road at any given time," states Marriott's press release. "Because hotel guests do not always see or interact with room attendants, their hard work is many times overlooked when it comes to tipping."

How much money should you leave? The American Hotel and Lodging Association, an industry trade group, recommends tipping housekeepers $1 to $5 a night, depending on the level of service and cost of the hotel. The Emily Post Institute concurs — its website recommends a tip of $2 to $5 a day.

Other important etiquette rules: Leave the tip every day, to ensure that whoever cleans the room that day gets the money. And be sure to put the cash in an envelope or leave a note next to the money saying "thanks" — any good housekeeper will be afraid to take cash if she's not sure it belongs to her.

Even though hotel bills are getting bigger, the people who clean the rooms still make a pittance. During the first half of 2014, travelers paid an average of $137 a night for hotels in the United States, up 5% from last year, according to Hotels.com. On average, maids and housekeepers in the traveler accommodation industry make just $21,800 a year, according to the Bureau of Labor Statistics — below the poverty line for a family of four.

Which leads some people to ask — why doesn't Marriott just pay its workers more, instead of asking customers to do it? For a $20.6 billion company , that's a fair question. But for now, if your manners compel you to tip the taxi driver, the bellhop, and the concierge, don't forget to leave a few bucks for the housekeeper, too.