Do a last minute check for everything before you leave for a show! I was at a show 3 hours away from home once and realized I forgot my hunt coat half an hour before my class.
Also, don't be afraid to over pack a little, and bring a travel vet box with you that has basics like bandages, vet wrap, a couple different kinds of medications, etc. in it. Never know when you'll need it!
First off, make sure that you and your horse both have the appropriate equipment and apparel for the show, double check everything with your trainer just to make sure. Also, practice the pattern of your opening and closing circles while jumping at home and really get a feel for your horses stride so you can tell when you will need to speed up or slow down in a line.
Hope that helped!! Good luck! :)