I start at 20% and it goes down from there based on how long it takes them to get me my menu and drinks, whether I have to hunt them down for a refill, if they're surly, and how they handle any complaints I may have about the food. I do not blame them or take it out on their tip when the food is substandard or cold but I do send it back and make the kitchen do it right. I would guess that the majority of the time, the waiters/waitresses get their full 20%.
There have been a few occasions when my friends and I have tied up the table longer than normal just because we were chatting and having a good time. In these cases, I always increase the tip by 15% to make up for the tip the waitress is probably losing by our tying up the table.
The highest I've ever tipped was when we took the entire family (50 people) to Olive Garden for my son's 16th birthday. Management told us that they automatically added a 15% tip to parties of this size and I just calmly said, "Fine...if that's all they want." Smart man did not add the tip and the waitresses ended up getting a 50% tip. What can I say. Only 3 waitresses dealing with us and none of us ever had to look for a refill, our meals came in a timely manner and they were hot.
Can you tell my mom did waitress work? As a general rule, these people work their butts off, get chewed out when the kitchen screws up (e.g. lose out on their tip) and go home at the end of the day with aching feet. To add insult to injury, if they are consistently undertipped, the federal tax laws are now set up so that they could actually end up paying taxes on money they never earned.