The thing that finally taught my mare to stand for mounting (she used to pretty much freak out anytime someone tried to get one) was I left her halter+lead rope on under her bridle, then hooked the reins on the saddle horn, took her to where I was going to mount at, and tried to get on.
If (when) she moved, I immediately set her away from me at a super fast trot, encouraging her on with my right hand and holding the lead rope with my left hand. Then, I'd ask her to stop.
If she stopped immediately, I tried to get on again (really, at first, all it took was me bouncing on the ground to get her to move off), and repeated this whole procedure if she moved when I went to get on again.
If she didn't stop immediately when I told her to, I'd keep pushing her on until she stopped as soon as I told her to.
At first, I tried this just out in her field but it made no impression on her. Then, I tried it on a gravel road and almost immediately she was like "Oh shoot, girl! Don't make me run on this gravel!" and she learned very quickly to stay put for mounting. I didn't stop having her halter (once I got on I liked to unclip the lead rope and leave it over the fence or whatever) on her for mounting until she was 100% at standing to be mounted. Now that she has this skill, I can get on from anywhere and expected her to be still but it was definitely a work in progress for a while!
I found that with Lacey, forcing her to stand still just got her more worked up and frantic about the whole situation. Making her move was what she wanted so I just made what she thought she wanted, not what she really wanted.