I am philosophically opposed to tipping. I think employers should pay a fair wage and not expect customers to subsidize their payroll.
That said, it is the custom and I'm not going to take my frustration out on the server.
I tip at least 15% at a restaurant - based on the entire bill including tax. More if the service was excellent. A little less if it was sub-standard. I tend to round down a bit if wine was involved - it irks me enough to pay $50 for a $12 bottle of wine.
$5 for the pizza guy, and the thai food delivery, etc. Maybe a bit more if it is a big order or the weather is crappy. All of these places are within 3 miles of the house. Same for a haircut, etc. 15 minutes of your time earns you a five spot from me.
When I travel (which is often), I carry a wad of singles for all of the folks with their hand outstretched - bellhops, shuttle drivers, etc. If you are obviously fishing for tips, you get a buck. If you are a little more subtle about it, you get more.
At the ballpark, I give the usher a buck per seat to wipe them off with a towel. I tip Beer Man generously, and he is very attentive. I tip generously at golf courses, because I know that many of those guys are working for tips and golf privileges - sometimes they don't even get a paycheck.
I was taught that you tip the help, but not the owner. My wife's hairdresser is the owner - he doesn't get a tip, but we bring a very nice wrapped gift for his family at the holidays.
The newspaper carrier gets a nice tip around the holidays, as does the guy who cuts my mother's lawn.
Unless there is a compelling reason, I don't tip counter help. The washroom attendant can kiss my ass (okay, not literally).
In short - I hate tipping, but I probably tip more than most people. I'm grateful that I don't have to depend on the generosity of others to pay my bills each month, so I try to do my part for those who do.