Facial expression

Become aware of the facial expressions you often make when stressed-out. If you raise your eyebrows often, you could very well end up with permanent wrinkles on your forehead. If you squint because you don't understand something, lines between your brows may become evident.
Don't expect to live without facial expressions, they're part of you. What you can do is be aware of those expressions you make when nobody is looking. Replace those with something else, such as fidgeting and ease up on your face.