Be careful. A perfectly innocent word in one country can mean something quite shocking in the other - e.g. some easily learnt differences between British and American English. Avoid slang, terse witty phrases or wisecracks or anything based on sport.  Jokes do not translate well. At best, they are likely to be confusing or misunderstood and at worst they may be taken as insults. 

For more information on use of words, see our pages on Classic Mistakes  and Language Services.  As regards the total content of what you may be saying, you must also keep in mind your own objectives and do nothing to diminish the likely receptivity of  your audience (see Attitudes and Expectations).
 

Body language 

It is very important to ensure that neither your actions, nor  your appearance nor your  posture could possibly be interpreted as in any way lacking in respect.

Complete attention should always be paid to the person with whom you are dealing. Eye contact ( remove your sun glasses) should normally be maintained and this is a particularly important in Europe, North and South America and Australasia. However, in a few African and Asian cultures (particularly Japan) eye contact can be disrespectful or disturbing, so be alert and prepared to break eye contact if it seems to be causing embarrassment. Equally, do not assume that someone who does not look you in the eye is untrustworthy.

 Posture avoid yawning, stretching or anything that might suggest distraction, boredom or disrespect. Do not slouch or stand with hands in pockets, hands on hips or arms folded. Do not  look around to see whether there are other people with whom you wish to make contact. If, after first encounter, conversation is continued while seated, sit up straight: and keep both feet on the ground..

Hand movements should be avoided so far as possible. Almost any gesture that can be thought of is likely to be obscene somewhere in the world. Gestures with the fingers should particularly be avoided. Some of them, such as pointing, the "thumbs up" sign and the "OK" sign (with thumb and forefinger forming a circle) can be very offensive. Muslims regard the left hand as unclean: food should not be touched with it. If handed anything by an Asian or Arab, receive it with both hands or with the left hand under the right wrist.

The head and upper parts of the body are often the most honored and the most private, so that it would be insulting to touch them. This is especially applicable to Asians, and you should aim, as a mark of respect to keep your head below the level of theirs (or, if this is physically impossible, to be seen to make some effort in that direction). To see a six foot man trying to  make polite conversation  over cocktails in Thailand  can be like watching a giraffe at a water hole, but good manners must follow the local rules.

The feet, by contrast, tare often considered unclean and demeaning, particularly in Asia and the Middle East. Do not move anything with the foot and never point the sole of the foot at another person. Should your foot accidentally touch someone, apologize profusely. When seated do not cross the legs: keep both feet on the floor.