Tipping really isn't that complicated. It's simple math. I've worked in restaurants (and yes it is hard work), so I always tip at least 20%. To calculate 20% quickly in your head, take the bill total and double it. Now just move the decimal point. For example: $20.00 bill x 2=$40.00, now move the decimal over and the tip is $4.00. Very easy, and a respectable tip so you won't have to feel uncomfortable.
Those of you who don't tip: Please never go out to eat ever again. In most states, servers are taxed for the tip, even if you don't leave one (in Michigan they are taxed for a 10% tip regardless). So when you stiff them on the tip, they are actually paying for it. Those of you using the defense that servers aren't forced to work in restaurants; well, you're stupid. You aren't forced to go to a restaurant. If you can't be bothered to tip, go pick up some fast food.