I agree with meupatdoes and others about treats. I LOVE using treats to teach standing at the mounting block.
However, with OTTBs, I don’t fuss and bounce and slither when getting on. They are used to a professional, light rider settling quietly in the saddle from a leg up. They are less tolerant of awkward straining, stirrup twisting, heaving aboard. It’s physically uncomfortable for them when you stand in the left stirrup for a length of time; it pulls the saddle onto the spine, digs tree point into the right shoulder, and makes the horse think mounting is unpleasant. I watched a video of a cowboy mounting from the ground WITH NO GIRTH, and he made it look easy. They key is to stand close to the horse’s shoulder, left hand on mane and reins, right hand NOT on the cantle but on the right saddle flap. Keep your center of gravity close to the saddle, kick your right leg high enough to clear the horse’s hip. When mounting racehorses (from a block, in an exercise saddle with half a tree) I get on quick, lightly settling into my knees off the horse’s back as I toe into the right stirrup.
Should a horse stand patiently as you scramble like a monkey? Sure that would be nice, but I want to make mounting easy and stress-free for my horses, so they never develop anxiety in the first place.