Some 'any help' for you: not buying a horse that's a known rearer+don't push her so far that she might rear?
I've found with my mare (who is a recovered rearer) that she'll start arching her neck down and sort of 'dancing' around, then she'll start rearing if I don't redirect her.
From Youtube videos I've seen of horses rearing undersaddle, it seems like the usual thing is throwing the head up, jigging a bit, THEN rearing.
I've found, with my rearer, that rearing is directly correlated to a refusal or inability to go forward.
For instance, if I hold the reins too tightly and don't give her a good release, or if there's something scary in front of us and she's not comfortable going forward, but I won't let her go backwards and I've tightened my grip on the reins (because I'm nervous).
I found that if I immediately loosen up the reins and urge her forward (or in a circle, or anything that gets her mind thinking about something other than the issue) I can generally get her out of that state of mind. That's actually how I "cured" her too - I redirected her every time she thought about rearing and now she'll usually redirect herself vs going up.
However, she's kind of like a recovering addict, I have to make sure I don't put her in situations where her only reasonable option is "up" because if she reaches the end of her ability to internally calm herself (through redirection and trying again) she WILL go up.