My horse is trained to walk up to the mounting block, line up and stand quietly until I say walk all through by clicker training. Check out this video to see how. It's a great lesson and skill for them to have in case you have a tall horse and get tossed on the trail!
Here's the "mounting a horse" video I made last week: YouTube - Mounting a horse.m4v
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So, all that's left is swinging your leg over, which I skipped here becasue I'm recovering from a concussion--(not horse related).
First... teach the horse to come to you when you stand up on something (a mounting block, a log, a big rock, a fence, etc). When he walks to you, click when you see his intention, then give a treat.
Move the block and see if he comes to you again. Then gradually withhold the click for position. Take it in small steps and eventually you will have a horse who comes to where you need him to be and stands quietly for mounting!
Email me for details if you want to know the steps to teach this. It only takes about an hour!