One quick tip for getting true and natural smiles from your models on any type of shoot is not to use the word ''Smile''. The quickest and simplest way is just to talk throughout the course of a shoot and be friendly. People will respond much better if you talk to them and make them feel at ease.
I normally don't shut up when I'm on a shoot and i'm constantly talking, even when I have the camera up to my eye. I'm just telling people my stories, finding out things about them, like things they like and don't like. Hell I even try and become a comedian at times. My jokes aren't funny, but It will evoke a natural response from the person I'm shooting when they laugh at how silly I sound.
I'm also constantly making a prat of myself. Normally by losing my balance or something along those lines. But I'm always ready to capture people when they least expect it.
Not everyone reacts the same way to one thing. But you will find that once of you have been shooting people for some time you will know what works and doesn't for specific people.
Model - Leigh Sinden