Basically, what you do is take an empty jar and put it somewhere visible, such as on a shelf or your desk. This jar is your "gratitude jar". Every time someone does something for you, or something happens for which you're grateful, you write it down on a note, perhaps with the date it happened, and store the note in the jar.
I've already brought one of my jars to school and I'm looking forward to doing this. I think it'll make me remember all kinds of little moments, compliments and encouragements I would otherwise forget, and it'll be a good feeling to see the jar with all the little notes in it. It'll also be a great lasting memory of my time at the school .