I can't remember in my head from year to year how much of everything we use on Pesach. How many pounds of ground meat, how many plastic cups, how many yogurts, etc. Or which brands were good/bad. So I used to end up overbuying a lot of things and still running short on some things for Pesach. This can get very expensive very quickly. Also while some things can be used after Pesach, some stuff just won't be touched and will end up being just a pricey waste.
Taking notes after Pesach for next year has saved me a lot of time money and aggravation. Before Pesach I prepare a shopping list based on last year's notes. I save a copy of this list and then update it with the actual things bought and actual quantities of each. 400 plastic forks, 12 cream cheeses, 4 rolls of aluminum foil, etc. At the end of Pesach I take notes of how much of everything actually got used and anything that was particularly good or bad. I also take note of lessons learned, things I want to do differently next year, anything that I want to add to kitchen tools, pots and pans, etc.
This sounds like a lot of work, but it's a small investment in time that pays off big to me next year.
What do you do at the end of Pesach to prepare for next year?