Watch the clip (courtesy of Centaur Biomechanics) of what happens when you mount from the ground. However agile you are you will inevitably pull the saddle to the left (if you are getting on from the near side), causing huge pressure peaks around the withers, and also pulling the horse off balance towards you. This can damage the horse’s back, and possibly the tree of the saddle! If you really have no other option, get someone to hold the opposite stirrup so that the saddle stays central.
Ideally, get on from a mounting block (the higher the better). Try and land lightly in the saddle. If your horse won’t line up next to your block, try positioning the horse between it and a wall or fence, so he can’t swing his quarters away. Train him to stand still, using an assistant and treats, until he learns to allow you to mount slowly, and stand still while you tighten the girth etc.
Can you vault on? This is better than climbing on from the ground, but does require spring in the riders’ knees….
Best of all – a leg-up, as long as rider and person doing the leg-up are in sync! The rider should not land on the horses neck, but be legged high enough that he or she can land lightly seated the saddle. Most racehorses are mounted this way on the move, and it takes plenty of time and practise to teach them to stand still.
Finally, make sure your girth is tight before you mount, not only to minimise the saddle being pulled off centre, but also backwards if the horse moves ‘mid mount’. Remember to check your girth again after you have walked a few yards.