Last year I kept a list on PAPER of everything that was in the freezer. This year I'm getting smarter about it. Write with a dry erase marker directly on the freezer/refrigerator doors. YES, even on stainless steel! That way it's truly easy to mark off what you use or add to it.
TIP: If you have more than one freezer & so that you know exactly what you have in one look, use one freezer/fridge like I did to mark exactly what's in each, instead of writing on both freezers.