I can't tell you how many times I've been involved in this conversation. With the blogging conference season entering full swing, the tips and traveling comes up in nearly every group I am in. So, I searched around the Internet and compiled a list. Now this is all according to the Internet, and it's not complete, but it covers the basics with a little extra on how much do you tip.
EVERY DAY LIFE
Food delivery person:* 10% of the bill (excl. tax), at least $1 for bills up to $10. Should tip 15%-20% for a difficult delivery. Don't ever assume that the driver gets a cut of the delivery fee many places tack on. Give a little more for bad weather, crappy neighborhoods etc.
Groceries: Check with store policy if tips are accepted. $1 for bringing bags to car; $1.50 to $3 if you have more than 3 bags.
Taxi driver: Varies depending on where you are. 15% is usually enough; and give an extra $1 to $2 for help with bags.
Barber: 15% to 20%, minimum $1, for a haircut. For other services (shampoo, shave or manicure) tip $1 to $2 to that person as well.
Hairdresser: 15% to 20%. (It is now acceptable to tip owner, unless he or she says otherwise.)
Shampoo person: $2
Spa service (e.g., massage, facial): 15% to 20%. If service is provided by owner, generally no tip.
Staff at coffee/food retailers with tip jars: No tip required. Some may argue otherwise, though I usually throw a buck in.
Waiter/waitress: 15% of bill (excl. tax) for average service; 20% for very good service; no less than 10% for poor service. Please don't skip the tip, your server may share tips with busboys or host staff.
Wine steward:15% of cost of the bottle and always ask before bringing your own bottle as many restaurants don't allow this!
Bartender: 15% to 20% of the tab, with a minimum of 50 cents per soda/juice etc, $1 per alcoholic drink. My feeling is even if you're sitting at the bar, drinking fizzy water and eating peanuts, you should still tip!
Coatroom attendant: $1 per coat or bag
Parking valet or garage attendant: $2 when they bring you your car
Skycap at airport: $1 per bag if you check-in curbside; $2 per bag if skycap takes bags to check-in counter. (This to me, so worth it, especially when traveling with kids)
Hotel doorman: $1 per bag for help with luggage; $1 per person for hailing a cab
Hotel bellhop: $1 per bag for bringing luggage to your room (but it's nice to give $2 if you just have one bag)
Hotel housekeeper: $2 to $5* per night (put in an envelope with a note so they know it's the tip and not random money)
Hotel concierge: $5 for getting you tickets or reservations (give them extra if they're hard to get). No tip required when you ask for directions.
Cruise: Varies. Ask about customary gratuities. Maybe some of my DisneyCruise blogger friends can chime in.
In addition, if you have a coupon for a service or restaurant, your tip is based on the before discount cost! So keep that in mind too!
And when tipping, saying Thank You is always nice too.