It depends on the level of show, and competition that regularly comes through.
Smaller schooling shows, are more relaxed, but a bigger show, you'll need to go all out.
If it's a small show, then just make sure she's clean, and tidy. Clean up her mane, 3-5 inches in length, so you can see her neck, clip legs, bridlepath, and muzzle. If you don't like clipping ears, at least clean them up and clip the stuff that sticks out. Well fitting halter that complements her face.
If it's a bigger show, you'll want to band the mane and forelock, clip more extensively (ears, muzzle, legs, clean up the top of the tail, etc). If she has a thin tail, tail extensions are recommended for a bigger show. Highlighters for the muzzle. You'll be seeing silver plated halters as well.
Main thing is, make sure she's clean, and looks ready and well put together. The judge will be looking at her in halter, so whatever shows off her strong points, highlight those.
"You're just as sane as I am."~Luna Lovegood.