If you've ever been confused about whether to tip (or how much), you're not alone. Here are some guidelines from Emily Post's Etiquette, tipping.org and industry resources:

• Airport workers:
  • Skycaps: Usually get $1 per bag.
  • Wheelchair attendants: $2-$5.
• Cabbies: 15 percent of fare.
• Coat check: $1 per item.
• Cruise ships: Check with the specific cruise line, as many of them have their own guidelines.
  • Cabin stewards, wait staff: $3 per day per person.
  • Busboys: $1.50 per day.
• Hotels:
  • Bellhops: $1 per bag.
  • Concierges: Between $2 and $10, depending on the service provided.
  • Courtesy shuttle drivers: At least $1 or more per person, or $4-$5 per party.
  • Door attendants: $1-$2 per bag, and $1-$3 for calling a cab.
  • Housekeepers: $1-$2 per night, left in a sealed marked envelope.
  • Room service: 15 percent of bill, unless gratuity already is part of the bill.
• Limo drivers: 20 percent of the bill.
• Restaurants:
  • Buffet-style wait staff: Five to 10 percent of the total bill, depending on the work the wait staff actually does.
  • Wait staff: 15 to 20 percent of the total bill — closer to the latter if the service is particularly good. However, if a wine steward picks and serves the wine, the wait staff's tip should be 15 to 20 percent of the food bill only. If you're dining in a group, check to be sure gratuity is not already part of the bill.
  • Bartenders: 10 to 15 percent of the bar tab.
  • Wine stewards (sommeliers): 15 to 20 percent of the wine bill.
• Tour guides and drivers: For extended tours, $1-$2 per day for each passenger; for day trips, $10 or 15 percent of the tour cost per passenger.
• Train personnel: Tipping is not expected on Amtrak trains; however, as a reward for good service:
  • Dining car waiters, stewards and bar car waiters: 15 percent of bill
  • Porters: $1 per bag
  • Sleeping car attendant: $5 per passenger per day
• Valet parking: $1, given when your car is returned.