1. Try to get someone not in the show to drive you there. This way you can get your hair show ready.
2. Get there early and start grooming and braiding ASAP. This way you have time to make you and your horse look perfect.
3. If you have a hot/energetic/hyper horse warm up is small circles. This is more work, gets them thinking more, and calms them down much more effectively. Knowing your horse is calm will most likely calm you down.
4. Any chance you get hop down and lean in your horse. Again this will help calm you down.
5. Always keep a hoof pick handy. You'd be surprised how many uses they have.
6. FLY SPRAY!!! Keep it handy at all times and apply a new layer every couple hours.
7. Invest in a horse hair soft brush. I recommend this one.
http://www.doversaddlery.com/winner's-circle®-ultra-soft-100%25-horsehair-brush/p/X1-1003/
It really enhances your horses natural shine. A bit of fly spray and spray on coat conditioner will also do wonders.
