Corn starch works better than baby powder, it's a bit more dense and stays better. Use it with a tooth brush & brush it into white legs. You can use it on the face too but I've found the easiest for face markings is a stick of chalk. Be sure to do white on legs before you use hoof polish if you plan to. Baby oil eyes, ears (inside & out) and muzzle. Clean udder/sheath and under the tail. A lot of folks forget that, it's one of the first things I look at when judging a showmanship class. Make sure all of your tack is clean and all of your keepers are used, especially on an english bridle.
Be sure you are clean & tidy. Keep a towel handy and wipe down your horse & your boots before you enter each class.
The most important thing, don't forget your smile and have fun!!!!
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.