I have never been a server at a restaurant, but I know many servers, and I have heard their horror stories of being shafted a tip at the end of a long dinner service. I also know most servers work full-time, even multiple jobs, and strive to provide great service, but are often times severely under tipped.
Just a few years ago, a multi-millionaire was shamed on our public news in Detroit for skipping out on a tip. He proclaimed he never knew any better, which seems to be the excuse of choice for non-tippers. Your server at a restaurant probably makes less than $2-4 an hour, and relies upon tips to survive. Not only does the server rely on those tips, but so do the bussers and bartenders, when the servers' tip them out at the end of the night, from the tips they collectively accumulated.
, someone skipped out on a tip by scribbling dumb imaginary rhetoric with religious undertones on their bill, and left 0% as a tip for the service he received. Unfortunately, servers cannot tell the customers that tips less than 15% of the total bill is a slap in their face, if they could, they would.
Great service is not free, nor is it included in the meal, your server provides that, and your tip pays for it. So how much should you tip? If you have a horrible dining experience, tip 10% and complain to the manager. If you had an average dinning experience, tip 15-20% (the standard), and if you had an exceptional dining experience, due to wonderful service, you can tip as much as you would like above 20%.
If you find yourself eating at a higher end restaurant, you might want to make 20% your average tip, as you are undoubtedly receiving professional service from your server. If you are a regular, the relationship with your server over the years can become very important, as they know your desires and preferences for service. I know servers who meet up with families they provide service for regularly, outside of the restaurant, because they enjoy each other's company, and have fostered personal relationships. That's how powerful great service by a server can be.
If a 15-20% tip seems high to you, you are eating out of your budget zone, and should eat at home, or at a food outlet that does not require a tip. There is never an excuse for not tipping your server; it is essentially stealing when you do not tip, and it is also morally corrupt.