Mind your thoughts
Chances are if you're feeling angry it is because you are having angry thoughts. If you are feeling sad it is because you are having sad thoughts. It is easy to become preoccupied with a set of thought patterns. A simple shift in thinking will have a dramatic effect on how you feel. I know it isn't as simple as that, but the principle remains. It is impossible to be happy all the time, however simple awareness of what is happening in our mind may enable us to shift our thinking and maybe alter our mood. I go back to the mindfulness clock. A great tool to help you stop and listen to your thoughts.