I have used both lungeing or 'sending' exercises to teach or retrain a horse to stand quietly to be mounted.
Essentially what you will be doing is making moving away from the mounting block, or the area you wish to get on the 'uncomfortable' thing to do, by making him work hard each time he goes to move away from you. I use a mounting block to teach the concept so he actually has something 'visual'.
I will go out, halter and lead on over his bridle (I use rope halters, so there is no interferance between it and the bridle should I decide to leave it on later when I ride, since it's not bulky). Go out to your mounting block, ask him to step up quietly beside it; at this stage you are only wanting him to step up quietly, you are not actually going to try to mount. If he does, just rub him all over...if he moves away, now is when you will put him to work. Immediately send him away from you...either to lunge, or do a sending exercise (which is just basically sending him back and forth in an arc in front of you, rather than in full circle. ) Make him really hustle his feet too, this is not lollygagging time...the quicker you can get him moving his feet, the quicker he will catch on in his mind that the place he will get rest is next to the mounting block! When you have done that for a few minutes, bring him back to the mounting block, and resume rubbing all over...no mounting, just getting him to realize it's an okay place to be, and there is no reason to anticipate when you are going to get on...make him think it could take all day, and eventually he will be falling asleep.
When he is comfortable being stopped infront of the block and rubbed (on both sides...always do both sides; since you have two sides of the brain to train), then start bringing him up to the block, and start stepping on to the block, and again, rub him all over, touching and tapping the saddle, moving stirrups etc...Rinse, wash, repeat; each time he sidles away, you put his feet to work...really focusing on making him hustle and pay attention to you.
When he is comfortable with that stage, then bring him in, rub, move the stirrups, etc, put a foot in the stirrup, step off, repeat... If at any point he moves...that's right, you put him to work. Eventually step on all the way, get off, step on, get off...you are reshaping his mind to not anticipate when you are going to get on. This will stick as well, too...my mare was a bucker bolter for mounting when I got her, and I did exactly what I have described, and she is super quiet for mounting now...could careless what I ask her to stand next to for me to mount up.
What you will wind up with is a horse who could careless about what you put him up to to mount, and will be patient for you TO mount...your horse may catch on in a 15 minute session, or you may have to periodically do this through out his life; some like to test, and we just have to be prepared to 'fight' back with what they already know. So teach it well, and he will never forget it. If my mare gets weird for some reason, all it takes is sending her back and forth 2 or 3 times and she's already searching and looking to come in to the mounting block!
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."