Back when he was president of the USA, George Bush Sr. visited Australia. From his caravan, he waved, saluted, and gave what he thought was the victory sign. The next day, all the local papers published the headlines "American President Insults Australian Public."
You see, in many countries, holding up your index and middle finger to make a V sign with the back of your hand facing away from you does not mean "victory" or "peace." It means "up yours." In fact in a couple countries in Europe, it doesn't matter which way your hand is facing, it's still an insult.
But you'd never do a silly thing like that, would you? Well, actually, you might. most Americans learn to count on their hands starting with their index finger, not using their thumb until they get to the number five. Try this and stop at the number two. See what I mean?
This is how, if you're not thinking about it, you're going to try to order two of something at a European or Aussie bar or restaurant and royally piss off your server. This isn't universal, just look at any photo that includes a Japanese (or Chinese or Korean) tourist. But it's an easy mistake to avoid.
So, while abroad, especially in Europe, Oz, and NZ, get in the habit of counting starting with your thumb (or, if you're weird like me, your pinky). After a while, it'll be as natural as measuring temperature in centigrade and using the metric system.
While we're on the subject of gestures, other obvious ones to avoid (or use, when the situation calls for it) are poking your forehead with your index finger or thumb, scratching under your chin with the back of your hand facing away from you, and of course the near-universal middle finger (with or without thumb-- you guys can argue that one along with your soda vs. pop debates, I need no part in the fight).
Beyond that, take a glance at what a guidebook says about gestures in your destination country. You'll learn how the finger wag North Americans use to tell their kids they've been bad simply means "no" in Latin America, and that nodding your head up and down in Bulgaria does as well. You might even be able to decipher the infamous Indian head wobble. If you do, let me know. None of the 25 students I was with or our (Indian-born) Hindi teacher could ever figure it out.