I tip based on a sliding scale of performance. Categories include friendliness, knowledge, bringing the correct amount of silverware and other such items (especially straws), how often they refill your drinks (if applicable), and other such job-related activities. Food preparation is never the waiters' fault, but how the problem is dealt with certainly is. My tips have ranged anywhere from direct complaints to the management resulting in the termination of the employee to a 100% tip. Typically I just round up to the nearest $5 increment, which is usually between 10 and 15%. 15% is the generally accepted standard, but mostly I consider it to be a starting number for an average job.
Edit: Tipping is not great if the tips go only to the wait staff. Your kitchen staff often feels slighted, and they actually have reason to make your food worse to lower the wait staff's tips. Nothing sucks more than waiters (or delivery drivers) bragging about $150 in tips when you just made $40 slaving away.
