Why any of this matters
Almost every session that runs late runs late for the same three reasons: an outfit that needed ironing, a drive that took longer than the map said, and hair or make-up starting half an hour after it should have. None of them are about photography, and all of them cost light.
This list ticks off in your own browser and stays there — nothing is sent to the studio. Print it if you would rather have it on paper the night before.
The three that matter most
Sleep, water and time. A session at the end of a bad night shows in the eyes more than any retouching can fix quietly; being dehydrated shows in the skin; and arriving with ten minutes to spare rather than ten minutes late changes the whole first half hour.
Outfits
Two or three you already love, on hangers rather than in a bag. If you are being photographed with other people, lay everything out together on a bed first — a shared palette reads far better than everyone matching. The guide to what to wear goes into it properly, colours, patterns and all.