In the United States it is customary to tip while receiving services. The theory of the tipping system is that you as the consumer have the power to decide how much they earn based on the service provided. If you feel they did very well, you would tip them as much as you would like to. However, if they provided poor service, you have the option to tip less. With this in mind, the service industry should provide the best service they can so that they may earn as much in tips as possible.
Compiled here is a general guide that will help with the different situations you may experience along with the averages of amounts usually tipped for each service.
(All rates valid as of January 12, 2015)
Every time you dine at a restaurant, you should tip the server 20% of your final bill. If service was horrible, you can tip lower. Do look on your bill as some restaurants may include a "Gratuity" charge already, especially if you are having a "set" meal, or have a certain amount of people. If there is a gratuity charge, no additional tip is necessary, unless you would like to give more.
Tip your bartender $1-$2 per drink that you order. If you are ordering a lot of drinks at once, the standard restaurant tipping system should be used.
The doorman will usually receive $1-$2 for getting you a taxi, and opening the door for you. If you are just arriving, storing your car and/or have baggage which he or she is helping you to retrieve from the car, he generally receives $.50-$1 per bag dollars for that service, since he will place it on a bell cart and bring it inside for you as well.
When luggage is stored, people generally do not tip. However when you go to retrieve your bags, it is customary to tip $1-$2 per bag and generally no more than $10. Giving the bellman $1 when asked to retrieve 3 bags is usually frowned upon; $2 is better if you are strapped for cash.
Room service usually has a gratuity included in the charge, check the receipt again carefully for this, if there is none, follow the guidelines of a restaurant. You may want to add a little extra if a delivery charge is not included.
Housekeepers check your room daily to make sure your beds are remade, the garbage is taken out, and the towels are re-hung or refreshed if you have asked them to be. In some hotels, your room may not have the same housekeeper every day. In this case, leaving the tip daily will ensure that each of your housekeepers receive a tip. A tip of usually $1-$5 per night, up to $10 a night, depending on the mess that was left is suggested. The tip is generally left on the desk or night table daily, or when you checkout; it can also be left at the front desk in an envelope marked "housekeeper of room (the room #)".
If the concierge helps you to book a restaurant that was hard to get, $5-$10 is appropriate; it may have been his or her connections or persistence that got the reservation made.
Generally no tip is given for checking in as none is expected, however if you were bugging the Front desk staff for something "Special" i.e. you had a Queen bedroom reserved and they were able to get you into a 2 Double room even though they are out of them, giving them a $5-$10 tip is a nice gesture but not necessary, the same guide works regarding complimentary upgrades.
Maintenance generally does not receive anything for fixing a broken light bulb or fixing anything in the room.
Request for items:
If you place a request for items, i.e. pillows, an iron, a fridge, a rollaway bed, etc.... The general guide is $1-$2 for small items, $3-$5 for larger items, with a minimum of $2.
Taxi drivers will generally receive 10%-20% of your total fare.