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Do you put exclamation points at the ends of most of your sentences? Be aware: in many parts of the world, doing that labels you an over-exuberant American - and, for some, a person to be avoided.
It's a mistake to deal with someone who has decent proficiency in English and assume that he or she is similar to you in every way. People raised in other cultures behave and think a bit unlike you - they're different, just as Americans are different - and it's essential that you remember that and not assume sameness.
Americans can be dismissive of educational credentials. But many other countries value educational achievement, and will want to know about your education before working with you.
Do you conduct sales conversations by asking questions? Be prepared for prospects in some cultures to find this uncomfortable. (Sometimes their discomfort is useful; if they're happy with the way they're handling whatever-it-is-you-do-for-a-living, why do they hesitate to answer? Could it be that they're really not comfortable with their current approach?)
Even something as simple as making eye contact varies by culture - an American may consider the lack of eye contact as evidence of "shiftiness," while people from parts of Central or Eastern Europe may find even small amounts of eye contact impertinent and rude.
Start to become aware of how much "idiomatic American English" you're speaking - and remember that many people who speak English in other parts of the world learned British English, and may find Americanisms unclear. Get them out of your conversation! (A few examples: references to baseball, such as "touch base," "off base," or "hit a home run"; Betty Crocker; or guns, such as "just shoot me," "take my shot," or even "oh, shoot!")
Be careful of judging another person negatively because he seems different from you. Instead, remember that people are about 99% the same, and be awake to differences that you can minimize in the name of making a warmer and more personal connection.