Whom to tip: Waiter
The going rate: 15 to 20 percent of the bill, pretax
Except when: You've had really terrible service. You can leave a smaller tip, but don't just stiff him and walk away, says Post. First, waiters are paid less than minimum wage, so your tip is expected (even bad service is still some service — you didn't pick up the food yourself). Second, you'll accomplish more by complaining to the manager, who will probably apologize and may offer you a discount on the check or a coupon.

Whom to tip: Coat check
The going rate: $1 per coat
Except when: You're attending a wedding or event in which the host would have paid for the service in advance. A good rule of thumb: If you see a tip jar, slip in a buck.

Whom to tip: Barista at the coffee shop
The going rate: No obligation
Except when: She has your daily mocha latte ready before you walk in the door — leave $1 or change whenever you can spare it.

Whom to tip: Restroom attendant
The going rate: 50 cents to $1
Except when: The attendant helps you with a grooming crisis, such as stopping a run in your panty hose; tip $2 or $3.

Whom to tip: Driver for car service to the airport
The going rate: 15 to 20 percent of fare plus 50 cents per bag
Except when: Gratuity has been included. Some car service companies do this, so check before you ride.

Whom to tip: Salon workers
The going rate: 15 to 20 percent of the bill for stylists, colorists and manicurists; $2 for shampoo
Except when: Your appointment was with the salon owner. Owners make a profit from the salon itself and don't expect tips.

Whom to tip: Delivery person
The going rate: $1 to $2; up to $5 if large order or bad weather
Except when: Your food was supposed to arrive in 20 minutes ... and an hour later you're still hungry. Call the restaurant first (it may have been a kitchen backup), then leave a
smaller tip. But if it's icy or there are torrential downpours outside, cut the guy some slack.