In psychological studies they have showed that people are usually very good at spotting a fake smile rather that a real one. This can of course be used if you want to use it in a sarcastic way. However, if you want to use it as a tool for receiving a pay rise or sexual favours you have to be able to fake it in a realistic way.
The aforementioned studies showed that it was the speed at which they appeared and faded, and the area around the eyes that gave it away. It seems as if there are some little muscles in that area that are not so easy to control consciously.
So what is the solution? One could be to go the face gym for a personal trainer, who would then let you spend years and money working on alternative muscle groups to make the illusion complete. Alternatively you can use my method.
Think of something that makes you smile.
Just a drive-by noding rescue attempt