I make camomile tea to pour over the feed of my stressy one. I also use flower remedies (rescue remedy usually). I also use magnesium oxide which is much cheaper than branded calmers.
I keep a bottle of Lavender & Chamomile in my grooming kit, massage a little into the horses neck/chest area (where they cant lick it off) before putting them in the box. Sometimes a little more when I get to the show, works wonders!
Used it on loads of different horses, always helps