We all have different features. The shape and the thickness of the chin majorly determine the appearance of the face. The general acceptance is that the subject should stick the chin out a little in order to appear nicer and sharper in the portrait. However, just sticking out the chin works only up to a certain extent. This is why you should think of something a little tricky.
Instead, you may try this tweaked trick.
As the photographer, you should position yourself slightly above the eye level of the subject. Now, you should ask the subject of the portrait to stick the chin out a little. After that, the subject must bring down the chin a little. However, this doesn’t mean that the subject should tilt the head down; instead, the subject should shift the entire head downwards (slightly). However, the subject mustn’t lower the jaw alone while maintaining this posture; it is all about sticking out the chin a little and shifting the head down. With a couple of tries, you will surely grab this trick correctly.