There is something fun about being in a foreign country and not being taken for an uninformed tourist. There is something sneaky and fun about confusing the locals and feeling as if you, too, were born and raised in Italy. If the locals aren't confused, at least it is possible to deceive other tourists. To have a tourist come up and ask you something about the city you are in is just divine.
Aim for an air of confidence. A certain confidence is necessary when trying to pull off the "I've lived in America my whole life, but at the flip of a switch can become a native Italian" look. If there is the least bit of fear or uncertainty, this is impossible. Don't smile too much, as it would appear as though you were enjoying the country like a tourist. Appear as though you know exactly where you are going and have something important to do once you get there.
Wear proper shoes to appear Italian. This is Italy, home of fancy shoes. Tennis shoes are out of the question. The statement that tennis shoes resonate is that you are an American and would rather be comfortable than wear cute shoes. An Italian could have two broken feet and ingrown toenails and still wear stylish, uncomfortable shoes on cobblestone.
Take care with your wardrobe. Baseball hats aren't allowed if you want to confuse the locals. Most Italians dress up every day. Americans have mastered the comfortable look, while Italians have mastered the art of looking nice even when running to the farmacia to get medicine when gravely ill. Older women will wear heels and dresses even when their ankles are swollen and they have nowhere to go. Jeans and T-shirts in Italy don't cut it. A good way to spruce up an American wardrobe would be to wear a scarf. Men and women alike can toss a scarf around their neck and change from John to Giovanni in a nanosecond.
Carry a pack of cigarettes. Smoking is bad for your health and while not condoning it, an easy way to appear Italian while sitting outside at a cafe sipping an espresso is to have a lighted cigarette in your hand. No smoking is necessary; just hold it nonchalantly and sip the strong brew. Any American walking down the street would immediately think, "Wow, look at that awesome Italian. I think I'll take a picture for my scrapbook."
Examine your hair color. Prior to embarking on a trip to Italy, if you have light-colored hair, invest in some darker dye and fix that problem. It will do wonders.