Congrats on your success so far and best of luck to you at the show. Here are two different ways I approach things, see if you think either one would work for you.
The first would be to approach it like you would any other show. Get into your routine and stay in it - both you and your horse will benefit from it. Memorize and walk your courses the same way you normally would, and warm-up and show the same way you normally would. You have made it this far, so you know your stuff. Just use it. (my routine - get to the barn and go work my horse, then bathe. Put him in the stall and feed him, get myself cleaned up then him. Go and memorize my patterns, warm-up, and show. My horse is so used to it he won't even look for food until after his bath
The other one is my own little way that works for me, but hey, I'm weird. I go to a couple of shows a year where there are 50 + horses in each of my classes, and multiple world and Congress champions in each class. I go in working my rear end off to do the best I can, but hey, I'm showing on a horse that cost me $1200 to buy vs $120,000 for some of the others. I'm not supossed to win. I'm just going to ride. This attitude for some reason works for me, and I put in my best rides of the year at these shows, and have won the circuit championships at these shows. (and then blown it the next weekend at a show I'm supossed to win at).
You will eventually figure out what works best for you, and just remember there is always next year. Let us know how the show goes for you.