Most people don't use their faces to their full potential, thus achieve atrophy.
You need to make sure to not have your jaw resting too far forward or two far backwords. even if you have slight over or under bite you want it to be more or less in the middle.
You can help train this by just trying to keep it in place and getting used to it and by doing certain mouth exercises like pretending to chew a huge piece of gum
Your lower jaw and chin are also important both from a practical point and from a visual point. One exercise you can use to tighten it up is to try to touch your nose with your tongue, you will actually feel those muscles stretching
You need to make sure your eyes are straight and aimed at the target, looking observing, not just collecting general details, you need to have them focused and it will improve the rest of your face. You also need to have your eyebrows work in tandem with your eyes and work properly, and not just be surprised or slacked at all times.