While those who provide services for us are paid for their job, it is customary to leave a tip to show your gratitude for their services.  My "boss"  told me a useful guide to some expected tip pricing when I was in the States.
Express your appreciation by looking the person in the eye when handing the tip. Also, don't use coin! Of course, if you find the service provided for you to be exceptional or poor, adjust appropriately.

HOTEL

Valet Parking

$1-2     The attendant should receive a tip each time s/he brings you your car. However, it is not necessary to tip again when you return the car.

$1      If your room is messy, you should give more than $1.

Bell Captain

$1      For each item of baggage, the Bell Captain should receive $1.

$2-3     For each person, $2-3 is acceptable. If s/he assists you with baggage or takes special steps to provide the best transportation for you, give more.

BARS & RESTAURANTS

Bartender

$1     If the drink requires mixing skills, tip $2. Otherwise, $1 per drink is sufficient.

Coat Check

$1     Give $1 per item you leave at the coat check, such as an umbrella or other jackets.

Diner Counter

10% - 15%    At least 10% of your bill is expected at the diner counter.

Restaurant Waiter

15% - 20%    As a general rule, 15% is adequate for satisfactory service. If you are really

happy with your server, feel free to tip more.