Last year, I had no filing system at all. Because a good alternative to having a bad filing system is to not have one at all, right? ;) (Good thing I only taught 3rd grade one year, because only about 5% of the resources I used made it out of that classroom without suffering death by recycling bin.) Luckily, one of my fabulous coworkers shared her filing system with me and it has absolutely saved my sanity!
It's seriously the most simplistic filing system ever. I have a folder for every week of the school year.
I keep each week's folder sitting on my back counter so I can easily throw in a copy of anything and everything we do that week.
I keep a copy of that week's plans in the folder as well as a master copy of everything I used throughout that week. That way, next year I will have everything I need to make copies of for that week in one place.
I also try to keep any samples I have modeled with, as well as student work samples, so I don't forget the "end result" of any projects/ craftivities.
I also throw in any extra supplies I have left over to save prep time for the following year.
Lastly, I baggy up any task cards I used and keep them in the folder so I can easily find them next year.
Obviously I will continue to add new ideas to my lessons each year, but this gives me a good starting point. It also makes it easy to remember what I've used in the past and helps me keep track of resources I choose to reuse.
I haven't done a great job of organizing the resources within the folders as far as content areas go, BUT at least everything is in one nice, neat place and not in a dumpster somewhere, right?! ;)