Watch this fascinating talk on Ted.com by Ron Gutman, entitled "The Hidden Power of the Smile:"
It's very difficult to frown when looking at someone who smiles. You ask why? Because is evolutionarily contagious and suppresses the control we usually have over our facial muscles.
From the Green Room: When you speak, your emotions are contagious. If you suffer during your presentation, your audience will suffer with you. But if you smile and your audience will smile in return - and instantly feel more positively towards you.