During my dialectical behaviour therapy training, one of the coping techniques I was shown was the half-smile. This involves relaxing your face and shoulders and slightly upturning the lips to form a small smile. This will send a message to your brain that you are happy and it will boost your mood. Studies have shown that when somebody uses a facial expression to enhance an emotion, their mood will start to align with the emotion that their face is communicating.
It will also have the added advantage that it will make you look more relaxed, more approachable and more confident!