It depends on whether you pay with cash or a credit card. If you pay with cash you just leave it on the table. If you pay with a credit card, there is a place on the slip where it can be added and then you tally up the total.

I didn't mean to sound harsh in my last post, its just that practices are different in different countries, and I do feel strongly if someone is going to visit another country they need to follow the practices of that country. As for tipping on take-out, the point of this thread, as you see from the replies, practices vary widely. But there is no question that if you are served food in a restaurant, it is expected you will tip 20% for satisfactory service.