A scientific study suggests that we respond most positively to people who smile slowly, according to this report by Psyblog.

A group of 100 men and women were asked to watch synthetic faces smiling in various different ways on a monitor and then make judgements according to 'how trustworthy, attractive, dominant, fake and flirtatious' the smiles made the faces seem.

The slower or 'long-onset smile' (0.5 sec) was deemed to be most authentic and flirtatious. 

Psyblog goes on to report:

"On top of this, the researchers found long-onset smiles were perceived as more attractive, more trustworthy and less dominant. Head tilting also increased attractiveness and trustworthiness but only if the head was tilted in the right direction. In this case, the right direction was the same way as eye orientation or towards a partner. "

 So there you go. If you want to instantly begin influencing those around you positively, slow down your smile and then tilt your head in the same direction that the person you are talking to is looking.