Everyday, I've decided to write something positive/funny on a note and stick it in a box. This can include events from a day out, funny conversations, encouraging messages from others, days where I've felt happier and so on.
Then, whenever I feel down or at the end of the year (for example), I can look at stuff written in said box and keep reminding myself why I'm still here.
Definitely won't cure my depression or other issues, but it'll keep me going just that little bit longer some time in the future.