There is one of these in every office - no doubt about it. This is the person that:
1.) Has had more exciting life experiences than anyone in the office
2.) Knows more about everything than anyone in the office
3.) Has heard of someone or knows someone who:
a. has had more exciting life experiences than anyone in the office
b. knows more about everything than anyone in the office
When you combine all of these traits, the result is clear - you will be one-upped each and every time.
You saw Niagra Falls once? That's cool, but he has a friend who saw a guy traverse it on a tight-rope.
You saw the Eiffel Tower last summer? Must have been nice, but she visited a friend in Paris who got them dinner reservations IN the Eiffel Tower.
You saw a huge accident on the way to work? Must have been crazy, but a year ago his friend saw four wrecks in the same morning.
You have two ways of dealing with a One-Upper:
1.) Stop telling stories around this person. Keep an eye out for the One-Upper while you are telling a story. If you see him coming near you while you are in the middle of a good story, wrap it up promptly then re-address it at a later time when the coast is clear.
2.) Intentionally tell stories around this person, wait to be inevitably one-upped, then one-up right back (even if you have to make it up). This may play out for several rounds until the One-Upper bows out. At this point, you can only hoped to have established yourself as THE storyteller in the office. Our One-Upper would dare not try to one-up you in the future, as he is certain of the end result. Just make sure you don't do this around other co-workers, as you will be seen as the new workplace One-Upper.