I’m also a “mature” AA and for schooling shows I wear white or light tan breeches and a conservative polo shirt: white, black or navy are the colors I have, but I have seen other shades of blue, dark green, burgundy. As long as it is a solid, conservative color, it looks quite nice. Because I am also pathologically incapable of staying clean in a horsey environment, I tend to prefer the tan breeches/colored shirt combo. Black belt, black boots, black helmet and either black gloves or (cooler for summer) those nice tan leather ones with the white crocheted backs.
I just twist my short ponytail into a low bun, pin a hairnet over my whole head to contain the inevitable wispies that escape, and plop my helmet on top.
Braiding makes my hands ache, so I skip it for schooling shows. A neatly pulled mane (or tidy longer mane for breeds traditionally kept long) is perfectly acceptable. The classic hunter braids are lovely and more than acceptable. If you want to be traditional, fewer, larger braids are more typical at dressage shows. Lots of people – myself included – like the easy-to-do button braids.
Otherwise: clean horse, clipped or unclipped to whatever degree you personally prefer, wearing clean tack, carrying a rider who is neat and conservative and also clean … more-or-less. :winkgrin:
Welcome to the dressage world; I hope you enjoy it!