Although I am an artist and a musician, I also have a very annoying habit of being super organized; my chorus teacher (I teach in a Jrk-12 school and he is in the classroom next door) always jokes that I know EXACTLY where every CD is that is in my collection! So.. here are a few things I try to do at the beginning of every school year... there are always a few things I really want to try every year so I keep a file in my cabinet called, "To Do".. I know, VERY original, dontcha think? Anyway, when I'm looking for something I want to try, or when I find a new song I want to arrange or an arrangement I want to try, I put a post it note on the page and keep it on a corner of my desk; during a free moment I'll photocopy it and stick it in the "To Do" file. In moments of boredom (yes, I get bored easily, too.. geez, aren't you finding out a lot about me today!), I'll take out one of the things to try and plan it into my next week of lessons. This way, I try to work through some new things every school year.
Another thing I've found useful is when I complete a lesson, in my plan book I write a quick note like "don't try this again next year" or "awesome" or "tweak to create B section next year". That way when I'm planning the next years lesson plans I can look back to see what worked well and what I need to change/add/adjust. As previously stated, I get bored easily so I don't teach the same material every year, but I DO teach some of the same lessons and have found certain songs/lessons that are gems and of course, try to improve upon them every year.
Hope this is helpful!