Putting on a happy and fun song, which instantly cheers you up. Try every time you feel sad.
Watching something funny and laughing.
Take deep long breaths and stretch your body and do some muscle relaxation.
Massage your temples with essential oils (I particularly prefer lavender and peppermint oil) The soothing fragrance actually relaxes your nerves and make you feel good. (you can use these oils if you have trouble sleeping also)
Try learning any new things or may be playing any new musical instruments.
Overpower your negative thoughts by only imagining positive things happening.
Do some cleaning. Coz less clutter in your house, less clutter in your brain.
Whatever is bothering you write it down on a paper and burn it. (It may sound stupid but has worked for me many a times).
Try and make a habit of reading feel good books (if you are not into reading). the books like 'The Art of Happiness' by Dalai Lama, 'The Authenticity Project' by Clare Pooley and many more such books which will genuinely make you feel better.
Talk to someone about it because nothing gives you more relief than sharing it with someone.