Talking about restaurants & service in general, although the service is already included in restaurant checks, you should leave some more money for the waiter on the table before you leave.
Dressing code: no matter how hot the weather is, you're not supposed to wear short pants or sleevless shirts or short skirts/dresses if you plan to visit a church, especially Saint Peter's.
Shopping on the street: when buying stuff from street vendors you can sometimes bargain the price, depending on the item and on the vendor. Of course knowing the local language will help you tons, since -as it happens in all touristic places- the more you look like a tourist the more they will try to raise the price.
To sum up, I can say I was very well received by all people in Rome, especially my friends, who were extremely kind and hospitable to me. Thank you guys, you are THE BEST!!. I miss you and I'll go back someday! On the picture you can see a VERY hospitable and warm Roman family receiving 2 foreigners with open arms...

