Now there is an app to make you smile.
Usually we smile because we are in a good mood, but holding a real-looking smile (even if you don't mean it!) can actually improve your mood: Smile For Real will make you hold that smile and take a photo of the evidence, which you can save or share if you like.
For best results turn the sound up, and keep your face well lit!
It works, it's free, and there no catch or marketing gimmick; If it cheers you up, suggest it to friends who you'd like to cheer up as well.
Also, try not smiling at it...