I have been using a little mental mantra drummed into me by my mother many, many, years ago. The original version was: Passport, ticket, boarding pass, money, jacket, bag, luggage (or receipt). Followed by anything else I might also have with me on that particular trip (e.g. once it was my pet hamster).

Nowadays ‘ticket’ and ‘boarding pass’ is of course replaced by phone, but the rest is pretty much the same.

I find that if repeat that little list (in my head, not aloud, I am not THAT old yet) at various critical points in the journey (security being the main one) then I am good. YMMV 😉

Just to add that the passport is the item I am most careful (paranoid) about, so as other have stated, best kept inside a larger, very visible item like a bag.