My husband and I have been doing this for a few years, and we absolutely love it. What you do is find a jar or container and a set of post it notes - throughout the year, when something good happens or something that makes you smile or you accomplished something, write it on a note and then fold it up and put it in the jar. Don't forget to add the date to the note. Then after the whole year passes on New Year's Eve, grab a glass of wine and open the notes together. You can remember all the happy things that happened to you this past year. We tend to forget all the happy things that happen throughout the year and focus on the bad or sad - so this is a great way to remember!
I have put notes in there about when a stranger commented about how beautiful I looked that day or when I found $1 in the parking lot.