TO some extent correct manners depend on the nature and address of a restaurant. Not to vociferously slurp approval of Japanese noodles in Osaka would betray the crudest behavior, while to repeat the ceremony in Paris might stir thoughts of the guillotine. Indigenous customs notwithstanding, there are rules of etiquette that are clearly universal. To be in command of them often requires nothing more than common sense. Perusing a magazine or other reading material at table, for example, is perfectly accep table at an outdoor cafe or casual coffee shop, but entirely inappropriate in more luxurious surroundings.
As soon as you are seated, open your napkin and place it on your lap, unless you are dining in a rustic setting, where you can in good faith tuck a napkin around your neck; the same gesture in better restaurants will brand you, at the very least, unsophisticated. If your napkin should slip forward, you may very well be able to measure a restaurant's professionalism by the promptness with which a waiter retrieves and replaces it. If no one materializes on the spot, ask a waiter for another napkin when he appears.
Similarly, if any piece of silverware should drop, do refrain from snorkeling; rather ask a waiter for a replacement. And feel free to request the removal of any dispensable item - table tents, intrusive candles, ashtrays, or swizzle sticks, for example - from the table.
Conversely, do not feel obliged to pick up or extend a plate or glass to help out a waiter unless you are asked to, as such behavior may be perceived as an insult to his professionalism.
I personally do not return food in a restaurant, even if the dish is noticeably overcooked. But it is perfectly acceptable to return food that does not meet any conditions you have specified.
It is also acceptable to ask for additional plates if diners wish to share a dish.
From `Elements of Etiquette: A Guide to Table Manners in an Imperfect World,' by Craig Claiborne (William Morrow and Company, Inc., c.1992 by Clam Shell Productions)