I agree with all the lady’s here that trying to look the best you can is not vain at all.
As a commercial photographer and videographer I know that posing can be difficult and so can smiling. Try saying something that ends in an easy EEEEE. Like Puppy’s or Money or Happy.
All of these words give you an nice easy smile with out your cheek bones rising up and making your eyes into little slits.
Try turning your body partially away from the camera so you look thinner.
There are so many posing tricks I would have to write a book.