...as in how to trigger and improve your reminiscence of a pleasant experience. (You could do the same for an unpleasant experience, but why?)
Certain stimuli are known to trigger reminiscence. The most well-known is scent. Decades ago, my first job was at a fence company. I spend hours in the shed out back, putting pointed tops on picket boards, using a specially-designed power tool as scary as any chainsaw used in a massacre. To this day, when I smell freshly-cut lumber, it instantly takes me back to that shed.
Then again, one of my first posts here noted how the scent of wood pervaded with incense smoke takes me to a Japanese temple.
Theoretically you can stimulate a reminscence by (1) identifying a relevant scent to the experience and then (2) deliberately exposing yourself to the scent later. The problem with this is that if you leave step (1) to chance, step (2) is likely to be hard to pull off. My suggestion is to artificially associate an unusual but easily obtainable scent with the initial experience, so that you can later expose yourself to the scent at will.
What kind of scent? Perhaps an herbal oil. These come in small convenient vials. But choose a scent you are not already overly familiar with.
I haven't tried this yet.
A second trigger for reminiscence, as everybody knows, is music. Everyone has songs that trigger memories of a certain time and place, or even a particular experience. Often the experience itself provides the musical trigger. Visit Disneyland, and when you come back you can find the ambient music loops on YouTube. Sometimes the connection is more complicated---I recently wrote about how a certain Quincy Jones song reminds me of walking in Tokyo at night.
But here again you can forge an artificial connection. On my trip to Myanmar some years ago (incredible bit of fortuitous timing), I made a point of listening to the Double song Rangoon Moon repeatedly. And now listening to it takes me back to that trip.
So next time you are planning a happy experience, design a soundtrack for it ahead of time. Then use your music player while the experience is happening, or on the way there and back.