I would argue that one cannot simply choose to be happy anymore than one can simply choose to be successful. Rather, like success, it comes by the consistent ability to make choices that lend themselves to happiness. When one makes choices that are in harmony with the nature of happiness, that person will come closer to that ideal. Anything else is simply momentary pleasure.
Some of the choices I’ve seen most consistently increase happiness include gratitude, service and love for others. Working towards our ideal self, becoming a better person tends to bring happiness. Progress equals happiness.
Gratitude in particular is a potent choice for happiness. It is a choice to be grateful– to focus on the things we a grateful for. We can make this choice often, even when we are sad.