I usually do 15% tip at sit-down, full-service restaurants (I should do 20% because it's easier math, but I'm an old dog and that's a new trick), but places like buffets (like FreshChoice) where there is no service, I don't leave anything. Delivery gets a small tip; haircuts, too, but a little larger. Pickup and coffee/beverage places don't get a tip from me because I feel I'm doing all the work. Bartenders ALWAYS get a tip of at least a buck. There's no better way to assure good, fast drinks than to keep your bartender/cocktail waitstaff happy. Plus, a few pops in me makes me pretty loose with the bills.
The trouble I have with tipping is in circumstances where I'm not paying for anything. We recently went to a one-year-old's b-day party and there was a clown (Sparkles). She made simple balloon animals and did some minimal face painting. I had to ask the host if we were supposed to tip her because I really had no clue (nor did I have any cash). The answer was a firm "no," because the clown was being paid $200 for the party (1 1/2 hours of "work"--I'm in the wrong field). But If I hadn't asked the host, I might have been inclined to slip Sparkles a twenty. I wonder what kind of balloon animals we would've gotten with a twenty...
And what about the baggers at grocery stores that help you push your cart to your car? I never have them help me, but my wife has occasionally needed their help when she has both kids and a ton of groceries. She'd never tip, but put in the same position, I might be inclined to tip a buck or two.
And how much for simple, free valet service? I say a buck.
Or at a parking garage where they're already reaming you for parking your car for an hour? Depends on my mood, but mostly it's a no.