Practice in your mind, with as much detail, before your show. You can even set up a little arena in your living room and pretend you are the horse. This is great exercise also!
When entering at A, create a square turn feeling and send your horse forward towards the judge. Be confident! This forward energy will also help you stay straight.
Exhale when you halt and salute, settle, take a deep breath again, and Go For It!
Ride into your corners and pay special attention to your geometry. During practice, try and have someone film you to help you review.
Breathe through each transition.
Have the slightest feeling of leg yielding out to help with your transitions. This will help keep you and your horse stay connected.
Steady on your outside rein.
Allow yourself to think of a marching song in your free walk. I like the Darth Vader theme from Star Wars, but find your favorite 4 beat cadence!
If something goes wrong, breathe and focus on what is next. Every moment counts. Don't let a past hiccup cloud the rest of your test!
Thank your horse for every stage of his/her learning. Appreciate all that you are learning together and Don't Be Too Hard On Yourself.