My tips on tips:
The often mentioned 15% is a great staring point for full table service at a moderate restaurant. I do 20% but that is a personal preference as we are usually repeat customers/regulars.
For buffet service move down a click (Cindy mentioned: 10-15%)
For inexpensive restaurants move up a click, to compensate for the fact that it takes similar effort to serve underpriced as overpriced food, say 20%.
Keep in mind the percenatge is for the price of the food (not taxes etc)
Also keep in mind for many places the tip is included (usually 18%) if your party is 6 or larger, that is: don't double tip.
Lastly, why would anyone stiff people who handle your food, behind closed doors? (You probably don't want to know what you have put in your mouth over the years... eew...)