Who are you supposed to tip?

I know that you are supposed to tip waiters and some hotel staff, but what other professions are you supposed to tip. I used to be a tour guide at Luray Caverns and only about 1 in a 100 would tip and I figured if people didn't know to tip tour guides, there must be other professions that get left out too.Who are you supposed to tip?
Maybe its hard to tell. Like a coffee shop for instance. I hate that the tip jar is next to the cash register all the time! I hate that I pay full price for whatever it is i'm buying and I hate that the ppl working are getting paid in full, and they still are expecting a tip.

I work in retail. I clean the store, bag the merchandise, take orders, deal with crappy ppl, etc etc, why can't I get tipped?
presidents

teachers

astronautsWho are you supposed to tip?
At airports the sky captains/luggage handlers

are people that should get tip(s).
delivery people and cab drivers get tips too

book clubs

carolina skiff