At the table, enjoy your company and the table setting and take the hints from the table setting. The role of the dinner napkin is especially important.
For the guests, as soon as the host unfolds his or her napkin you should follow.
Place your napkin on your lap. Either completely unfolded the napkin or lay it out in half lengthwise. (Here I have been making my napkin like a mini tablecloth across my lap!) There it remains for the meal.
Leaving the table? Place your napkin on your chair. This is the signal that you are returning.
Then again, the host signals, with his or her napkin, that the meal is over by placing their napkin on the table. You follow by placing your napkin on the table to the right of your plate. Not a refold and not a tight wad either just place it to the right of the plate.
As Margaret Visser notes, in her very interesting book The Rituals of Dinner ".. in most countries the unfolded napkin shows that you know your host will wash it, and not give it to someone else, and that you do not think you are to stay on for a second meal". The one exception is that napkin rings, for family meals, means that the napkin is returned to the ring as it is very appropriate to use a family napkin for another meal.
What about those napkin rings? Martha Stewart says that they impart a "ceremonious sense of opening the meal". We like them also.