People always say 'breathe'. But when you try to do that, you may just be gulping in air and holding your breath which won't help! So instead focus on breathing out - slow and steady - instead. Works for me.
Also proper preparation and knowing where you're meant to be and when - and allowing a little extra time in case of scr3w-ups - and you'll be fine.
yep i had to take rescue remedy when i did my first dressage test! i was SO nervous! lol! i think the more you do it the less you feel nervous!
thats the theory anyway!