To practice mounting, one breaks down the components into tiny steps and practice several times each.
First, stand at mounting block. Reward, walk away. Repeat, repeat, repeat.
2nd horse at mounting block, rider fiddles with stirrups, girth, whatever. Reward walk away
3rd horse at mounting block, rider puts weight on saddle or stirrup, reward walk away
4th horse at mounting block, rider sits on horse and gets right back off. Reward, walk away
Each time go through each step, like building blocks.
A similar process to this was very helpful to me with my mare, who wasn't really trying to completely run away, but she would not stand still to be mounted and then had a bad habit of trying to walk off while I was still trying to get my right foot in the stirrup, which for me was an unacceptable combo because I'm a newbie and it just made me more and more nervous, which in turn made her more and more nervous.
I combined this process with clicker training.
First I taught her to associate a specific sound I make with getting a treat. Then when I'd ask her to do something really simple, (like stop or back up when walking her, lower her head while I had the bridle in my hand, stand still to put the saddle on, etc. stuff she already knew how to do) and she did those things right, I'd make the sound and give her a treat. So that she knew to associate that sound with giving the correct answer and with being rewarded.
Then I moved on to putting weight in the stirrup with my hand, and when she would stand still, clicker noise and treat. After being able to repeat that successfully a number of times, I'd put my toe on the stirrup and if she stood still, clicker noise and treat. Then on to putting weight in the stirrup, springing up like I was going to mount, and so forth until I could put myself belly-over the saddle and she would stand still then I'd make the noise, get down and give her a treat.
From there, being able to be laying over the saddle with her standing still, I taught her that when I made the noise she could turn her head to the right to receive a treat while I was on her back. Once she could do that, it was a fairly short transition to being able to put my leg over and sit in the saddle and when I would give the noise cue she would bend her head to the right and take a treat while I put my right foot in the stirrup.
Now we are working on being rewarded when she stops and stands still when asked.
We worked on this for months and have been very successful so far, but the first thing was teaching her the noise associated with the treat.
If you have a horse that gets pushy over treats, a good way to deal with that is to ONLY give them the positive clicker noise and a treat when they aren't trying to mug you for a treat. For example, when I was teaching her what the noise meant I would ONLY make the noise and give her a treat once she had stopped nosing me for treats and put her head back in her own space. I was doubtful at the beginning that it was going to work because she used to be so pushy about treats, but using that method also made her less pushy about treats and food in general.