Just smile with a genuine smile.
It is not only the process when we curve our lips to make smiling face; it is not the gesture of politeness. But the way we smile is the indication of our fairness: wide or narrow smile, with closed or opened lips, with shining or freezing glance.

From the Physiology point of view, while smiling our body releases endorphins that send message to brain and causes confidence… satisfaction.

You give just smile and get positive emotions, satisfaction, happiness, proud, and not to say about anything else you just get smile.

There are several types of smiles: smile with closed lips that means that it is only politeness, smile with whole face, identified as genuine smile, and covering the mouth.

Smile is a very important element and comprises 3 secrets of buddy language:
As far as women are shorter for 5.5 inches than men, the best way to make woman say “yes” is to keep your eyes below women. But “leaning in” early in the conversation is definitely “No.” The guarantee of comfort between men and woman conversation is eye contact rather than looking down.

The most debated woman smile in the history of art is the smile of Leonardo da Vinci ‘s (1452-1519), young Florentine (Italy) woman, Mona Lisa.

According to the http://www.ehow.com/ smile works at work too, and gives several instructions to go ahead easier with a smiling face. To be smiley at work follow the instructions bellow:

Some bad experience? It should be resulted not by bad mood but with a smile trying to determine what the problem is.

Take time to realize the problem.

Help others to solve problems rather than be involved into these obstacles.

Are u\you a manager? You should make face-to-face communication with team members to exclude the problems.

And if the work seems boring, like you are stuck at one and the same place without any progress, analyze your opportunities and options that will make you be more smiley even at current job.

References:
http://www.kevinhogan.com/bodylanguage.html
http://www.bodylanguagesignals.com/Mouth.html
http://www.ehow.com/how_2091117_keep-smiling-work.html?ref=fuel