Every part of the world is different, so these tips will certainly not apply to all cultures.  I happen to be from Central Asia, so these customs will certainly apply to that area.  Iran and the Middle East have similar customs and many of these apply to Russians as well.

When entering a house, remove your shoes

When we first bought our house, a salesman from a security company came in with boots.  As we stared at his boots on our beautiful new hardwood floors he noticed that and responded - "don't worry, I wiped them first".  We did not buy anything.  Note that most people provide slippers for guests in the entry.  For us real estate agents, this is probably obvious.

If you are male, always shake the hands of other men in the room upon entering and leaving

I still have to tell my husband to do this.  Not doing so is a mark of disrespect and considered rude.

If you cannot make it to an event, call to say you are not coming

Most people from my country come very late to events, but if they say they'll be there they come.  If they cannot make it they call.  I have noticed that when many people are invited, Americans will often say they are coming and then never show up.  This is extremely rude, as in my culture it is custom to not serve the food until all guests are accounted for.

If you are a man, when dealing with a foreign couple do not speak too much directly to the wife

Most likely, the wife is truly the decision maker, but may not say very much.  She will listen carefully to what you say to her husband, but addressing her directly too often will be considered rude by both the husband and the wife.

When food is served, try to eat at least a little bit

Not trying the food is considered very rude since a lot of time was likely spent preparing it.