In my backpacking days, I made a complete list of everything that I might possibly want to pack. I would go through it methodically when getting ready for a trip, both to be sure that I had what I needed and also to eliminate any extra items and weight. It was really a lot of work but very successful. I never found myself on the trail lacking anything.
These days I am lucky to remember wallet, cellphone, keys and glasses when I leave the house.