I'd get a competition bag and pack the night of the show or about 3 days ahead.
I like having a bag of essentials, as well as others bags for other important things.
I have a Kensington all purpose saddle bag to keep all of my horse's tack in
Put thing in boxes/ bags a nd label them
I try to have different coloured bins for everything I need and then I also have them labeled so I know where everything is and where to put it etc.
Have a small tubber wear bin for braiding supplies !! It gets lost so easiey hahaah. And. A garbage bag for saddle pad
Make two lists, one for you and one for your horse. When you pack, check off everything that you have packed. Then, once you get to the show, check again. Then again when you leave.
Also different boxes for different things (ex: XC, Show-Jump, First Aid, extra tack, extra equipment, etc,)