Tell her to smile or laugh while saying the vowels out loud. (Go ahead. do it in the mirror and see what I mean.) Not only does this get different expressions but it allows for photos where it looks like the model is smilling and talking to someone.
Fashion Photography is not a simple job as it seems, the creativity of complete freedom with new ideas are important to apply in the method of work, which requires complete training and some sort of inspiration to select professional photo shoot.