What kind of shows have you been doing before? Have you been to a show for that long? (I assume if you're doing HITS you'll be going at least a week, maybe more?). The biggest difference between the longer show series and the more local stuff is, obviously, length of time. So the stuff you need increases dramatically. Not sure where you'll be staying but I never stay in places that have washers/dryers. Which means that bring I at least 5 or 6 pairs of show breeches and show shirts depending on how many days my classes are. It's a bit of a pain packing because you'll need all of your show clothes, clothes to get dirty in around the barn and schooling, and nicer clothes to wear if you go out to dinner or whatnot.
As far as packing horse stuff, I'd assume your trainer/barn is in charge of all horse care items (buckets, feed, curtains, etc) so hopefully you won't have to worry about all of that. I'd invest in a good trunk if you don't have one already. By good I don't necessarily mean expensive (although there are a ton of really nice ones at HITS!), but one that's big and sturdy to carry all of your stuff. Besides all of your show tack you'll also need to bring schooling tack (if it's separate), stuff to clean it, etc. I always bring a few show pads. I'm a little picky but after showing a few horses, they always get nasty fairly quickly. Don't forget a seam ripper, as you'll most likely have to rebraid your horse a few times throughout the show. I really think a set of tack carrying bags (i forget the official terminology: bridle bags, garment bags -a must-, saddle carriers, etc) are very very helpful. And you'll need to wrap your horse at night so don't forget your no-bows and standing wraps (if that's what you guys do). Fortunatley the bigger shows always have huge tack venders where you can buy stuff if you forget anything.
I also bring lots of snacks with me (although most of the parents at our barn deck out our barn aisle with baskets of food!). You never know with show food, sometimes it's expensive, sometimes it's not great, although I did go to a show last summer where they provided dinner almost every night! Don't forget stuff to do for when you're waiting around. I love watching other classes but there are times when it's nice to sit down with a book and chill a while.
So I'm not sure if that's what you meant by "preparing" for the big shows but making sure you have everything is always good in my book! There's definitely a different atmosphere at the bigger shows but I love it! The competition is tougher and rounds that could win at a smaller show sometimes won't even ribbon at some of the bigger ones. I just spent a week at a show and was ecstatic about an 8th place ribbon. I was going to do the 2nd week but my horse is young and green and he was awesome so I changed my mind and scratched to give him the week off. The key is to just do your best and have fun! Good luck and keep us updated!!