You just tip. That's how it is.
You tip 10% in Europe, 15% in the US. (I don't know about other places). It
doesn't make sense, but you also can't not tip, because you're probably not
going to be able to explain to your server all your thoughts about tipping.
If I could change everyone's views about tipping, I'd go with something close
to Dan's views: the server gets paid in full by the restaurant and the
customer tips if the service is extraordinary.
If you don't like that, then we can have a system where a certain percentage
tip is standart, and the food prices are lower because the restaurant doesn't
pay the serving staff as much.
And about the chefs, I think it makes the most sense to tip the people who
prepare your food. I've heard that it's customary to tip higher than usual in
sushi bars because preparing sushi is hard and you're tipping the person who's
doing it.