I use a checklist app on my phone. Either ever note or keep notes. I use voice to text to create the checklists so that I don't need to type anything. Then anytime, anywhere, if something pops into my head that I should add to the list, I can just get my phone out and add it. Both of these apps have a an option to use a checkbox in front of every item. So when I'm packing, I can just check the boxes.
I can copy and paste a checklist from a prior trip into a new note and modify it if needed. It's pretty handy