Stretching the neck, to the point that it hurts at the back of their low neck, pushing the chin out or forward will help eliminate the double or sagging chin without the need for you to shoot from very high up and angled down.
Pushing the shoulder closest to the camera down, and leaning toward camera will also elongate the neck and help avoid a double-chin situation. You will also notice that posing your client like that, at the same time putting their weight on the back foot and bending their forward knee, makes them look a lot slimmer.
Lying on their stomach or leaning forward onto some surface will also give you a variety of great tips for boudoir photography poses when they can support their chin and hide double chin, where they can look up at you which also helps with stretching their neck. You can arch their back, lift their legs and cross them in the playful manner and, of occurs, hide the stomach area.