A tip is a payment, usually in cash or change, left for a waiter or waitress to show gratitude for the service rendered. The tip you leave should be based off the quality of this service and not the cost of the meal. When it comes to tipping your server there are a few things to keep in mind to help influence the amount you leave.

Here are things to think about while tipping some one working in food service:
This is not the time waiting for your food, your server has no control over how long the cook takes to prepare it.
When you sat down at your table, keep in mind how long it takes for you waiter or waitress to get with you. If they are rushed, give them a bit of extra time to help you.
If you can see your food when it is finished, watch how long it takes for it to be brought to you. A server's main duty is to do this.
How often does he check on how you are doing? When he passes by your table, even if he is doing something else, he can at least ask if you need something.
When the waiter or waitress talks to you is he or she about the sell, the service, or conversation?
A server's job is to sell, they must do this. If they fail to do this, there is no need for a tip.
Being polite is a key factor for any service job. When they serve you, a good waiter or waitress should make you feel good about being there.
A waiter or waitress that attempts actual conversation is a great thing to have. Telling jokes, giving personal views, and suggesting meal items is a very inviting way to wait a table. A server being honest and open helps also. If you ask how the soup is today, and he replies with something to the effect of "trust me you don't want it" they saved you from spending money on something that would upset you.
Servers should be able to distribute your food and create an enjoyable dining experience.
Does your server clean your table of dirty plates when you have finished? How about wiping up those spills?
When something is wrong, does your server fix it, or at least attempt to do so? If you order mustard on you burger, does he get it? Was it his fault that it wasn't there in the first place?
Does your waiter or waitress make you feel welcome? When you get your bill, do you get the impression you should leave or do you feel welcome to sit and chill for a bit?
What about the cost of the food? If you find certain item that you ordered off the bill, he may have not forgotten but instead given you free food. In this case, they has gone out of their way, risking their employment, to give you something extra. All such items should increase the service charge you give them. (I also suggest a thank you, but avoid saying anything about, their boss may be around.)
And remember, it takes just as much work to serve a $20 steak as it does a $1 sausage biscuit.