Seth Godin writes, “When you’re having a good day, go for a walk and record a ten minute audio sharing your optimism, confidence and possibility. You’ll want to listen to it again.” 

If you know you’ll want to remember it later, then give your future self a prompt. That way, you can also practice the memory right as you need it—before an important conversation, as you start a new project, etc.

Make a happiness folder, or a map of memories.