The first rule is that lunch matters more than dinner. Book the table for half past twelve and do not plan anything for the afternoon.
Bread, then everything else
The second rule is to buy your bread before it runs out. The bakery in San Quirico opens at seven and is often empty by noon.
Order what the table next to you is having. It is always better than what is on the menu.
That is the third rule, and the one visitors always break. Nobody here will mind if you ask.