We all know most of our servers at our favorite eatery are severely underpaid and truly do survive on the tips and gratuity of their customers. Most food servers are making far less than minimum wage - normally a little over $2 an hour!
It's not like a normal 9-5 job where you can clock in and coast and earn the same pay as when you put in a hard day's work. A food server is at the mercy of the customer.
Most people know a 15% - 20% tip is the norm, but realize what you're tipping for.
Normally a tip will go directly to the server and other times they will split w/ the bus person and other assisting staff. The cooks normally don't receive any part of the tip. So if you're lowering your tip because the food was not as palatable as you had hoped but the service was excellent then I'd suggest reconsidering.

Speak with the manager if the food was poor, but don't lower your tip because of it.
While you're contemplating what you'll be tipping next time you're out, think of what you'd expect and do your best to be fair and reasonable. Face it, it's better than having to wait on yourself at home!