Sing. Really sing. Belt it out. To the rafters. (For example, at a karaoke or similarly appropriate place). Give it all you've got. It's good for breathing deeper, too.
At home, put on some music, and sing along. Don't worry about your skill. Don't be too shy. Just let it all out. Happy it all out. (Happy as a verb? Why not.)
If you're in a place where you can't sing loudly, then sing low. Or hum. Hum under your breath. Even for just a moment. Any cheerful tune. You can make one up. If you like. Just say Tra-la-la-la-la, and go on from there. No need to put too much effort into it. Let it happy out naturally.
Try to sing or hum or whistle a little, every day. It's very uplifting.
Any pleasant song or tune. The more positive the better. For instance, I know someone who loves to sing or hum Christmas Carols like "Jingle Bells" and "Deck the halls", at any time of the year... The lyrics and maybe even a sing-along version can be found on the internet.
NB. Whether singing or putting on the radio, TV, music system, etc., let's always be considerate of the people, children, and pets in the same house, or next door, or in the neighbourhood. Even during the daytime or weekend. Since we ourselves wouldn't like to be disturbed, let's not disturb others. Keep the volume low, and try to use headphones as often as possible. Cheers.
