Keep a diary and note down your feelings, your emotions, at different times of the day – morning, afternoon, evening, night.

Do this for about a month. Then take a look at your diary. Watch the patterns that your mood follows. Understand your mood cycle.

The next time you feel a low mood, remind yourself of the pattern in your emotional cycle. Remind yourself that this present mood is just one part of that cycle and
enjoy it. It is, that’s all. But it is not forever, it will pass.

In the same way, if you are feeling really happy, watch. Watch the emotion, witness
yourself through it.

Practice this technique for a few weeks. You will see that you are able to handle your emotions without getting disturbed, no matter what emotion comes up at each moment.

source: Living Enlightenment