Most of my life I have worked in some sort of service industry, specifically in food and beverage. I have held positions of lowly dishwasher, busboy, pizza-maker, expediter, server, bartender, manger, and general manager. During my 10 plus years in the industry, there is one constant that I have seen in EVERY position (except kitchens), PEOPLE DON'T KNOW HOW TO TIP!
IF you don't know, servers and bartenders typically make under $3 an hour, and thus DEPEND on their tips to survive. Also, in most restaurants the servers have to tip out on their sales, meaning they have to "tip" the busboy's, bartenders, and food runners - I actually had a table where I lost money as the server, the bill was $500 and I got $12 tip (French people) and I had to tip out my busboy based on sales which came to $15 on that particular table!
The typical percentage to tip is 20%; when I say typical, I mean this is what the servers expect. Now, when I say expect, this is if proper service is given. I know there are times when you don't receive the proper service that you expect, and in this particular situation then it is understandable that the tip be affected - if the reason for the poor service is the server's fault. For example, if your food comes out incorrectly or cooked wrong, most likely this is not the fault of the server, but the kitchen. The server's tip should not be affected in this situation, they have no control over the kitchen, or the 'ticket times' once they are rang in.
Please keep in mind the type of restaurant that you are dining at when thinking about the service. If you are in a Denny's or Ihop, don't expect your server to be "all over your table", in those restaurants the servers have huge sections (# of tables in their section) and have to take care of a large number of guests. Whereas, if you are dining in a fine restaurant, the servers usually have only 4-5 tables to take care of, greatly decreasing the number of guests they are taking care of.
Many people enjoy wine with their dinner, and feel that if a bottle is ordered that they shouldn't tip on that amount (in the case of an over $100 purchase). Again, keep the service in mind; does your server keep your glasses full, do you ever have to poor your own wine, was the wine decanted, did the server make a wonderful recommendation for your meal - all these things should be in mind when tipping at the end of the meal. Not to say you should tip 20% on a $500 bottle of wine, but you should tip some sort of amount, at least 10 - 15% (on wine only, the rest of the bill should be 20%).
So please, keep your server in mind at the end of the meal, they depend on their tips. I can guarantee, that if you frequent a restaurant and get known for being a bad tipper, you will receive poor service every time.