Today we took a step by step approach (did you see what I did there?) to mounting with Davy who has recently started to move around when his owner gets on. With the aid of my mobile mounting block we had the problem solved in no time at all. One of my answers for Horsescene Magazine was in this subject:
Horses may begin to fidget at the mounting block for a variety of reasons - because they are experiencing or have experienced pain when someone has mounted, because they have not been taught to stand quietly or, for example with racehorses, because they have always been mounted on the move.
Before training your horse to stand still for mounting, it's really important to rule out any physical cause and it is worth having your horse's back checked and making sure that his saddle fits well and is placed properly. Consider the girth too.
It's much better for your horse's back if you can always use a mounting block to mount from and I find that a mobile mounting block of a decent height is better than an static one. Where the mounting block cannot be moved, it is really easy for the horse to just take one step away with his hindquarters to thwart your attempts to get on. With a mobile mounting block you can move it with him so that he doesn’t gain anything. You can even reward the horse for standing still by moving the mounting block away when he does stand and repeat this a few times.
In the same way you can then lift your leg up against his side and if he stands, take it away again and repeat this a few times. When you finally prepare to mount, think about holding the offside rein slightly more tightly as if you pull on the inside rein, you will inadvertently turn his bottom away from you; you could also ask someone to quietly hold him while you get on. Once mounted, ask him to stand for a good 30 seconds or so and be as relaxed as you can - remember to breathe! This will teach him that he doesn’t have to move off the instant you are on board. Give him a lovely rub to reward him for standing still.
If you apply this approach consistently and never overlook the mounting problem because it isn’t your priority for the day, you should find that he gets better and better.