I'm doing this. It's a good practice just to stop at the end of the day and think of something good that happened, there are a few days that I wasn't able to put anything in the jar as well...hmmm.
I bought myself a page a day calendar (half off) so I always have a little bit of paper handy to write on. And I got a beautiful tall glass jar with one of those clamp tops at the flea market for a buck!