WillowNightWind, PLEASE read my post, especially the part regarding flexing your horse to the right. That piece of advice is very dangerous.
Pick your mounting apart piece by piece. For my method, it's best to do this in a rope halter with a 12-14 foot lead, do NOT tie your lead rope like reins. You can use split reins if you horse lunges extremelly well. If you use split reins, half hitch your right rein over your saddle horn a couple times to secure it.
Flex his head slightly to the left as you prepare to mount. By flexing to the left, if he does move, his hindquarters will move away from you. If you flex to the right ( away from you), it is a matter of time before you try it on a horse that you are very seriously injured on. If your horse moves, the option that is left to them is to run over the top of you, or if you're trying to mount on a rank horse, get kicked. If a horse pulls any attitude, you want to be able to drive their hindquarters AWAY from you. If they really try to move, that rein will be snatch out of your hand as well. Do NOT flex to the opposite side to mount.
Then find at what point does he move. Can get you up next to him? Can you place your left hand on his mane and the right on your saddle horn? Can you raise your left leg? Can you put your foot in the stirrup etc? When he stands still, retreat for a moment and give him a rub on the neck. By retreating and giving a rub, you are reinforcing that is what you want.Then try again and go a bit farther.
You will come across the time where he is no longer wanting to stand still. At this point, send him off pretty darn firmly. Point to the left and with your right hand, use the tail end of your lead rope to pop him on the shoulder to get him moving around you, then on his hindquarters to drive him forward faster. After a couple cirlces, ask him to stop and try again. You HAVE to get him moving ASAP. If more than a couple seconds pass, he will not connect the two behaviors.
Effective horse training is all about letting your horse make choices and then making those choices easy or difficult depending on what you want from him. He wants to move around and go forward, so let him. Really drive him forward and let him commit to that choice. "Do you really want to move around? Awesome, moving around is fun, lets do it!" The fact of the matter is that he doesn't really want to move forward with that much energy. So you then give him that opportunity to stand still again and rub on him. You really want to make standing a pretty good option.
NEVER get on him if he's so much as thinking about moving. Only mount if he's standing calmly. If you mount while he's moving, you aren't really teaching anything. After you mount, stand still for a few moments. The biggest reason that horses move when mounting is because they anticipate moving off as soon as you're up there. If they anticipate standing still, that works out great. The next biggest reason is riders not mounting well. If you pull on him to mount and make him unbalanced, of course he is going to wiggle around.
Horses get this after just a couple of tries because it's ridiculously easy for them to understand. The important thing to remember is that you have to make the option that you want, standing, the better option. If you just run your horse around, or lunge them for more than a couple of circles, they won't get it. They'll only know what you get next to them and then all of a sudden expect them to run around you. If you only flex a horse that's really wanting to move, you aren't really teaching that horse to decide to WANT to stand still.