Former child model and actress here. (Shoot me.)
Practice. Practice in front of the camera. Practice with selfies. Find the angle you like most in the mirror and learn how to tilt your head to recreate it. Always note where the light source is, angle your face toward it and lengthen your neck. Try a bunch of different smiles/mouth shapes and see which feels the most natural. I've found I'm a close-mouthed smiler, a la the Olsen twins, myself. Also it helps if I think of something pleasant so my eyes don't look dead. Tyra taught me the importance of smizing.
Take your photos in natural light and avoid shooting from below up (it's rarely too flattering on anyone). Shoot in diffused natural light (partially cloudy afternoons are great). And find interesting ways of framing the shot so it's not just your face in the middle of the frame.
And then Facetune, of which I am not ashamed.
And then practice some more.