If you are not needing to thin her mane in order for it to lay properly, and have a few weeks before a show. I just get some thinning scissors and cut the mane in layers. The trick is to try it out on a horse you don't show first in case of any mistakes, and do it a few weeks before the show so it can look more natural.
I prefer the thinning scissors when I go to the local shows. Big shows may be another deal.