Adjusting your posture is one of those things.
Also, give this little exercise a go for a week and see if you notice a pattern: when you feel happy - how do your neck and shoulder feel? Probably relaxed and flexible. How do you hold your head? Now try to observe yourself when you are feeling cross or stresses? Tight? Holding your breath or not breathing deeply? Head hunched forward? Take a deep breath (breathe with your tummy, not your shoulders), exhale, repeat and relax. Do you feel the tension easing?
Keep observing your body and if you feel yourself tensing up take a deep breath and check your posture - it will soon become second nature to you!