Keep a journal. Does't have to be anything deep, just a list of any out of the ordinary activities you did on a given day. Things like movies you saw, dates you went on, conversations you had with friends etc.

I started doing this a few years ago and it's really great at the end of the to look back year and see all the cool things I did.

There are a few apps for this. I use Everday.me a bit and have been meaning to try Overdrive.

There's also a company called OhLife that sends you a daily email to reply to. I think they're YC.