There's no faster way to create a neon sign over your head that says 'TOURIST' than to commit a cultural faux pas. Unfortunately, missteps in a foreign country may also end up seriously offending someone or giving the impression that you are rude. In America it's commonplace to eat french fries with your fingers, give someone the thumbs up, or hand an item to a co-worker with your left hand. Doing the same things in some other countries would earn you stares of disbelief.
Many faux pas seem to revolve around cultural customs related to food and drink. In Japan one should never stick their chopsticks upright in their rice bowl. In funeral services a bowl of rice with upright chopsticks is placed in front of the casket of the deceased. It's extremely offensive for this to be done at any other time. So where do the chopsticks go when you're not eating? In front of you, side-by-side, parallel to the table. In the Middle East and India food is not to be eaten with your left hand. The left hand is considered dirty because it is used for bodily hygiene. If you are a left-handed person, you should not use your right hand for anything. Bread usually accompanies a meal in France, but the bread doesn't belong on your plate. It belongs directly on the tablecloth. It also isn't eaten before the meal, but rather with your meal or with dessert. It can never hurt to take a look around the dinner table to see what everyone else is doing.
Hand gestures can also create some awkward situations. Giving the 'okay' sign in Brazil or Germany is incredibly insulting, like calling someone a name or giving them the middle finger. You could also be flipping the bird by giving someone in Greece, Russia, or Latin America a 'thumbs-up.' Beckoning someone in the Philippines by curling your index finger gives the impression you think that person is inferior, as this motion is only used to call dogs. So be careful about how you're "talking" with your hands.
It's important (and also rather easy) to learn some of the common mistakes travelers make when visiting foreign countries. Then you don't make the same mistakes. Know what is offensive and unacceptable before you even land on their ground. You're a guest, and you should want to make a good impression. Acknowledging customs and manners will go a long way to ensuring you a happy and safe trip.