William Moss of Moss in the City - a fantastic little e-newsletter I get from the National Gardening Association about urban and small space gardening - had a great article about reviewing your year in gardening. It's a great suggestion, and I confess that geek that I am I keep a garden journal - complete with diagrams, thank you very much - that catalogs what I plant and roughly when, how it goes throughout the season, and an end of the season wrap-up. It's nothing fancy - just a notebook with graph paper marked pages for easy diagramming - but it helps me keep track of what's happening. It's also fun to look back and see what worked, what didn't quite go as planned, and just remember.
I've done this since my first garden at the first place we lived here in Michigan, and it has helped me rotate crops (or at least know when I have NOT rotated crops), remember what I grew that I liked, how it did in a particular location, and what pairs I created.