Mounting Manners Matter

Shiloh's expression shows discomfort as her saddle is pulled off-center

Mounting manners matter – peoples’ manners, that is! A rider’s weight puts enormous torque on a horse’s spine, especially when mounting from the ground. It can not only make horses uncomfortable, as Shiloh’s expression shows in this photo, it can damage the spine in multiple ways. (See References below.) This can lead to back pain and “behavior” and “training" problems such as bucking, balking, and refusing jumps.

Shiloh is more relaxed here

Use a mounting block or get a leg up whenever possible. Hold mane and pommel of saddle to reduce slipping. Holding the back of the saddle makes it twist. Sit down like you are sitting on eggs. Ideally an assistant would hold the off-side stirrup leather to counter Sarah's weight as she mounts. Being considerate about mounting gets your ride off to a positive start.

Bronzz's and my favorite mounting block

My overturned (rusty) stock tank cost nothing, and I can easily swing my leg over without even using the stirrup. It is safer to put your foot in the stirrup first in case the horse moves, so the choice may depend on your horse's mounting manners.
For more tips on being a rider horses want to carry, see What Horses Really Want Chapter 19.

References:
Ease Mounting Pressures on Your Horse https://equusmagazine.com/horse-care/ease-mounting-pressures-on-your-horse-8326/
The Importance of Mounting Blocks https://holistichorse.com/health-care/the-importance-of-mounting-blocks/