The first real question is: "When am I expected to leave someone a tip?" The answer is that you are expected to leave a tip for someone if they are providing you a service for which the tip they are earning is part of their wage. Everywhere you go these days you see tipping cans and buckets, but most of the people looking for these tips are people who earn a paycheck anyway, and tipping is OPTIONAL, but not expected or required. If you go into a restaurant and sit down and someone waits on you, your family, and your friends, you are expected to leave a tip, so read on!
If you receive average service in a restaurant, you should leave a tip of 15-18% of the check. If you have had items taken off of your bill or discounts from coupons, you should be tipping on the original bill, because you were provided with service for the larger amount. Average service would include a server who was friendly and efficient, available for your needs, and prompt. A server should usually not be blamed for errors by the kitchen, unless it is blatantly obvious that it was his or her fault for errors with the order.
If you are blessed with excellent service in a restaurant, you should return the favor and leave a tip of 20% or more to show your server that he or she really took care of you! Working in a restaurant is hard, hard work. Pleasing the many different personalities that show up in a restaurant everyday is a challenge for most people in the restaurant industry, and I assure you, they do feel appreciated when they see a big tip for their efforts!
Poor service happens, too, I am sad to say. In all of the restaurants I have worked in, I can point out who was the worst server in the bunch. Some people just don't get it. In this situation, I ask you to do what your heart tells you, but in my humble opinion, 10% tells your server that they were really bad.
I feel it is important in conclusion to mention that tipping in a restaurant is actually not required. It is EXPECTED, and there is certainly a difference. Not to get back on my soap box, but if someone serves you food or drink and you decide to leave them a less than standard tip, you are actually telling the server something about their service. If they are a good server they expect to receive 15-20% in gratuity, and if you don't plan on leaving that kind of a tip, you should think about going to an establishment where tipping is not expected.