In restaurants, 15-20 percent for expected service, more for extra good service and/or an exceptionally pleasant server. Even for a snack like a slice of pie and coffee, I'll leave a 5 buck minimum tip. When the service is poor, I'll watch to see if the server is overworked because of a rush of customers or trying to take up the slack for a missing server. If the server is busting their tail just to keep up, I'll still leave them a normal tip, but if they're goofing off, talking on a cell phone, etc., the tip will be much less. Nothing burns my fanny more than having to wait while a clerk or server is gossiping with their friends on a cell phone. There's only been a very few times I thought service was bad enough to not leave any tip and always because the server was goofing off and not doing their job.
The barber charges 7 bucks for a haircut and I give him 10. It's an old fashioned barbershop with all the typical BS, local gossip, political debate, and friendly banter. My rural mail carrier gets a 50 buck gift certificate to one of her favorite restaurants every year for Christmas. You can set your clock by her deliveries and she goes above and beyond with all the package deliveries I receive, even to the point of tracking me down to sign for packages when I'm not home but somewhere close on her route..she knows all my hangouts. Always brings the larger packages up to the house and never leaves anything out in the weather like the idiots from UPS and FedEx.