Found this on the web:
Generally, parking valets should be tipped $1-2 when you leave your car and again $1-$2 when they get your car. Originally, tips were optional and given to reward exceptional service.
Nowadays, tips are more or less expected. Many service workers are paid minimum wage, and rely on tips to augment their earnings.
Here are some guidelines for tipping: Generally, salaried staff such as hotel managers or cruise captains are not tipped. Gratuities are given to service employees such as valets, waiters, housekeepers, taxicab drivers, airport porters, hairstylists, and tour guides.
Always carry smaller bills such as one-dollar bills (or two-dollar bills) when going out to dinner or traveling. Except for gratuities added to your credit card, or those left in the hotel room, or in a note as indicated below, its nice to fold the bill at least in half and say thank you while handing the gratuity to them.
The amount of the tip is up to you. But, the following are some general gratuity amounts:
AT THE RESTAURANT:
Maitre d': $20 or more, if a special service is performed such as getting you a table when you have no reservation and the restaurant is full.
Waiters: 15-20% of the bill, unless a gratuity is already added to the bill. In the United States normally a gratuity is not automatically added unless you have a party of 6 or more. In a buffet restaurant, add 10% to the bill for gratuity. Many foreign countries automatically add gratuities, so check your bill carefully. Overseas, even if the gratuity is already added, some people like to leave some small change on the table.
Sommelier (Wine Steward): 15% percent of the bottle price
Cocktail Waitress or Bar Waiter: 15% of bill or $1 minimum whichever is greater (i.e. if a drink costs $5, then 15% is 75 cents, but leave $1).
Bartender: if you are served at the bar, 15% of the bill, or $1 minimum, whichever is greater.
Coat Check: $1 per coat
Restroom Attendants: $0.50-$1
Musician in Lounge: $1-$5.
AT THE HOTEL:
Parking Valets: $1-2 when you leave your car and again when they get your car.
Bell Hop: $1 per bag plus a couple extra if he shows you the room.
Doorman: $1 for hailing a cab; if he helps with luggage, same as Bell Hop.
Concierge: nothing for simple questions. But, if they make restaurant reservations, obtain theater tickets for you, make travel arrangements, then $5-$10 per task performed. Put the gratuity in an envelope with a note of thanks and give to the concierge.
Room Service: 15% of the bill, unless a gratuity is already added, then no additional tip or $1.
Delivery to Room: if you requested something delivered such as a hairdryer: $1
Housekeepers (maids): $1-2 per day left at the end of your stay. (I leave around $1 per day per person and I leave it each day, makes more sense to take advantage of the goodwill while you are there)
Spa Services: 15-20%, if a service charge is not already included. Ask that the gratuity be divided among the providers.
Swimming Pool Attendant: No tip for handing out towels, only if special service performed, then $1-$2.
AT THE AIRPORT:
Porters and Skycaps: $1 per bag
Shoeshine Person: $3-$5
Cab Drivers: 15% of the bill.
ON A CRUISE SHIP:
Some cruise lines add the gratuity onto the payment for you, so check with the specific cruise ship on which you will be traveling. Otherwise, plan to include tipping as part of the cost of taking a cruise. Usually small envelopes are left in your cabin with suggestions for gratuity amounts such as:
Cabin Stewards and Waiters: $3-$4 each per guest per day.
Maitre d' and Busboy: $1.50-$2 each per guest per day. (Some suggest $10-$15 to Maitre d' for the week).
Hand out the gratuities in the envelopes to each person at the last evenings meal, and to the cabin steward the day before the cruise ends. On a cruise ship, you don't need to tip your bartender, wine steward or bar waiter because a 15% surcharge has been added to every drink for gratuity. Some cruise lines may state tipping not required, but tips should still be given for good service. The link below gives tipping guidelines for different cruise lines.
OTHER SERVICES PROVIDED:
Tour Guide: $2-$5 for a 1-2 hour sightseeing tour.
Pizza Delivery: at $1 per pizza or 10% of the bill.
Furniture Delivery: $10 each person
Hair stylist: 15% of the bill, whichever is more.
Blackjack Dealer: $2-$10 or minimum bet at the table, after you're through playing (click on link below for more details)
Change Person at Slot Machine: 5-10% of your jackpot.