Don't stop to think!
Go with the first things that comes to mind:
What are three things that make you smile?
Great! Take a deep breath.
And that's how fast you can improve your mood. Never forget that if it feels easy and breezy then it's working for you. Too often we feel something has to be hard or take a long time to be effective. It's kind of engrained in our minds if something didn't take a long time and a lot of effort it's not worth squat. Forget that thinking! Banish it! Things can be easy and you deserve for them to be so.
Ready for one more?
Think of a time you laughed REALLY hard.
Awesome. Well done. Enjoy your day and keep thinking of things that make you smile and laugh.