Ghost of a smile.
A smile can hide a thousand emotions, you just have to practice.
That's right come on, lift your head.... That's it. Relax your shoulders. If you're tense it makes the smile unbelievable and stiff. Now try to smile. Try pulling the corners of your mouth up more but not too much - if need be show some teeth and give a chuckle while you're doing it. If you practice long enough you can learn to keep this facade up all day.
The eyes are often the downfall though - a real smile can be seen by the emotion in the eyes. My advice is focus on the area under their eyes or even their shoulder. Never focus on their eyes unless you are sure you can decieve them. It takes a lot of time to be able to decieve someone while maintaining eye contact.
I have perfected this smile.
This smile protects me from the hurt that others inflict.
I can wear this like a cloak and discard it in the solitude of my home.
It's perfect.
It's useful
It's.... so
Lonely.