Hopefully she won't do it undersaddle. I have tons and tons of advice for working with rearing horses, given to me by old-timers, found in books and training videos, off the web, from trainers, etc. The thing about rearing is that it is the most dangerous thing a horse can do, especially if you are on their back.
If you are on the ground, the biggest thing is to prevent the behavior before it happens by keeping the horse moving and keeping her attention. People will tell you to flip the horse over but so many times that can go terribly wrong and injure the handler or kill the horse, I do not advise that.
When you are on the horse, you want to keep it moving forward, if the horse is moving forward they can't rear. Do tight circles, serpentines, spins, and lots of forward movement. Always keep her mind on you and what you are planning to do. Same with on the ground, keep her eyes on you. Have her always wondering what you are going to do next.
I have a rearer. I bought him as a 3 year old that already reared. I've tried everything. He is 25 and still rears. Some horses can be cured, but rearing... is the worse.
Good luck to you, keep us informed... I'll be interested to hear what you do and how it works.
"Be a best friend, tell the truth, and overuse I love you
Go to work, do your best, don't outsmart your common sense
Never let your prayin knees get lazy
And love like crazy"