It's just the polite thing to do and a shankss for a job well done!
So here's the 2cents (no pun intended) with tipping people, not cows.
airport skycap: $1 to $3 a bag
food delivery: 10 to 15% of the bill
shuttle-bus driver: up to $2
waiter: 15% to 20% of the bill
-note: when splitting a bill, include the 20% tip of your portion of the bill into the $ "pile" -
bar tender: 15% to 20% of the tab, with a minimum of 50 cents per soft drink, $1 per alcoholic drink
cab driver: 15% plus a $1 or $2 for help with bags
Spa service: 15-20%
(ps: I didn't put in the rules for "doormen" or "dog walkers" because most of us don't have that...yet)
(specifics from realsimple)