Why do you all pack so late?? The day before surely that makes you more stressed?? I've got all my clothes already on my case and I've got 3 weeks left just a matter of maybe a week before to tidy it up a little! I like to make sure I have everything sorted!
Pack the night before. I normally have a rough idea of the clothes I want to take. If I forgot something, it gives me an excuse to buy a replacement. Also, we have the cat problem as well where they "play up" if they see the cases.
The only thing I might pre-organise, is the piling up of electronic stuff and related chargers.