If you feel like you're missing things, or actually missing them, take some time on a day you're at the barn but not riding (like a rainy day), and make a list of all the things you will need to pack for a show. Save the list on your computer. Print out a new copy every time you have a show and check things off as you pack them.
If you show in many shows all around the same time as each other, keep logs of each show that keep track of your paperwork. Put the name of the show at the top of the page, list the classes you want to enter, and keep a tally of all the fees for that show, when you sent off paperwork and of your times.
On a different page, keep track of the classes you're entering so you know if you need to work on specific aspects more.
That's all I could think of without specific questions.