I'm going to tell you about the American way to tip someone. You'd think this is simple/no brainer, but you'd be surprised working as a server as to how many people don't know how to tip.
First off, a little history lesson on it.
When the tip was first "invented" God knows when, tip stood for "To Insure Promptness" meaning if a patron who is trying to show someone a good time, whether it be clients or close friends, came into a bar and handed the bartender a 20 spot, it was communication to the bartender saying: "Hey treat me better than your normal patron"
Since then the tip has become a little more required. It varies from state to state, but since the tip has been assimilated into our culture, our employers and government have become savvy to that. Employers are now legally allowed to pay us less than minimum per hour because the tips servers make will make up for the wages they're not paying to make minimum wage. The government even taxes the tips we make (on paper anyway, shhh) because tips alone can for sure make the difference between tiers of income that uncle sam uses to tax people.
The tip can be a gift and a curse at the same time. On the good side, you are leaving your place of employment with cash in hand every day. On the bad side, you are leaving your place of employment with cash in hand every day. The reason its bad because it destroys budgeting skills because the server gets the 'oh I'll make enough to cover expenses tomorrow'.
The solution: DONT COUNT ON MONEY YOU HAVENT MADE.... EVER
On the other side, the tip is an extra expense for good work. A server is prompt, talkative, funny, and engaging. He/she is obviously working for the 20+% tip. The problem is that its up to YOU to give him that tip. To put it simply, back in the day, tips were given before hand so the server will treat them before everyone else. Say a gentleman wants you to get him an orange, he offers you $5; then say another gentleman wants the same orange, but he offers you $20. Who will you get the orange to first? A true capitalist would get the 2nd gentleman an orange first.
But nowadays a tip is merely used as a 'review' of the service that is given. The problem is that everyones definition of a good tip is totally different.
A server that is funny, personable, engaging and prompt might get $10 from one family, but $25 from another. Its dependent on so many variables like average home income, mood, service, RACE(yes race, on the giving and receiving end) that it makes it a challenge to keep a steady tip percentage. Some people cap their tip at 10%, some at 20%, but some servers who actually go above and beyond can get 50-100% (I've seen it happen). Show someone the best 1-2 hours of their life and they'll hand you their life savings, its how things work, your time might be worth more or less to certain people.
Some people don't even know better, I'm not trying to offend anyone, considering its the way they were raised, but people from out of the USA. They don't tip as much as we do, not even Canada does! In Germany, this 'tip' is actually given for beer money or considered an insult. They tend to give 5-10% according to Wiki (http://en.wikipedia.org/wiki/Tip_%28gratuity%29)
If you happen upon the perfect server, please realize they're jumping through so many hoops for you that they're mentally and physically exhausted at the end of the day and probably do more work in 5 hours than you do in your 9-5 day, and tip them accordingly. We are not the scum of the earth, we are not there because nobody else will take us, were some of the more brilliant minds of the world, we think fast, walk fast, and act fast, all while 1000 things are going through our head.
I hope this will help you in your next restaurant adventure and I hope your server works as hard as I do.