This may vary between one person and another. For me, there are several things that can makes me feel as if I can suddenly feel the way I feel in the past. There are indeed certain things that can bring me back to past. Those the things that remind me about some memories, those things that have really strong connection with the emotion I ever had before.
The first thing that can bring back the memories is ofcourse the photographs. I could not agree more. So here is my advice, collect every pictures of us, may be there are still a lot of photos in our friends social network account or something. Dont wait until we get lost contact with them, someday, we'll regret it when we are get separated from them.
