I face this challenge, too. I think a checklist is necessary until I form a habit that I consider to be reliable. I haven't forgotten my keys or wallet yet, but I almost have a few times, and it makes me nervous.

The best way to use a checklist is to print out many copies and to actually make check marks with a pen when each thing is properly packed. Often when I look at a checklist, my eyes jump over an item, and I say "yup" to myself, but I haven't actually done everything on the list. The safest thing is requiring myself to mark each item with a pen and not to leave until all items are marked.