My mare came to me with a bucking problem undersaddle, as well as a 'leap foward' as soon as you try to put foot in stirrup. Here's what I did...
One, thing I made sure to do was to do solid ground work with her for probably 2-3 weeks before even considering getting on.
When I started putting the saddle on her, that's all I did at first (for about two days); put it on, longe her, do some bending, and then strip it off her, and that was that.
Then I started taking the stirrups and starting slapping the fenders on her sides, as well as shaking them foward and back; this was to get her 'reused' to 'leg movement' on her, without putting myself at risk of her tossing me...Lol! I did this on both sides...not stopping until she was relaxed, or standing still for atleast 10 seconds. I then moved onto putting my hand in the stirrup and putting some weight there. I, again, did not 'release' the pressure until she was quiet.
To get her reused to someone being 'above her' I starting utilizing her 'send command' to send her between a fence and a barrel, which I was perched on. When she was comfortable going back and forth with me sitting on the barrel, I then stood up and did the same thing. When she was going 'through' without panicking, I then would stop her at an area between the barrel and the fence, so that I could slap on her saddle, pet her, etc.
Once she was comfortable with the whole send, stop, and be rubbed\saddle slapped on, I then moved onto leaning over her. No foot in stirrup yet...I just got her used to me simply leaning over her a bit. Patted and petted her all over on the side that I was leaned over on. When she was fully comforable with that, I moved onto putting the foot in the stirrup, and putting my weight on her that way.
I would then mount up, get off, mount up, get off, etc. I never quit while she was upset, or not standing still. I also never proceded into getting on if she wasn't totally trusting and relaxed in what I was doing with her...get on while she was in 'flight' mode, was asking for her to revert back to her previous behaviors.
I think at one point she had either tossed someone, or had been jabbed hard in the side while someone was mounting, and this caused her initial behavior; the previous owners who had her, didn't know how to work through the behavior, so it set deeper in (I believe they bought her not knowing she had this behavior). Anyway, she is now able to be ridden, and even after a few weeks of not being ridden, I got on her today with a BOOST from my hubby...something I had NOT done with her prior...she stood like a ROCK!!!! I was really proud of her!
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."