This sign is extremely offensive in Brazil!
In the U.S. this is the peace sign, in the U.K. and Australia it can get you into trouble.
It's Not All Rock 'n' Roll to Them
The devil’s horns means “rock 'n' roll” in the U.S., but in Italy it means a person's spouse is unfaithful!
Unlucky for Some
Fingers crossed for luck is OK in most places, but it won't fly in Vietnam.
It might be a positive sign to you, but keep your thumbs to yourself in Iran, Greece, and Afghanistan.
Take a moment to pick up the phone and call clients and vendors overseas. It will help you understand the tone, and English level, of the person you are working with.
When you're invited to coffee, it is important to go, even if you're a tea drinker. Many international contacts will want to get to know you on a personal level.
Be aware of differences in personal interaction. Take your cues on how close to stand, and how much eye contact to use, from your international colleagues.
Before you travel, get to know about local passions. In South Africa, Australia, and New Zealand, rugby is huge; in Europe and South America, soccer rules.