When my gelding or mare gets a whim and decides not to stand for mounting, I immediately turn him/her in a circle for about 5 minutes (I know, makes your head dizzy). And, I'm not talking about a leisurely stroll here, I mean to get his hindquarters moving and work a sweat. Stop and then try to mount. If he stands, great. If not, back to the circling. After a few times, he gets it and stands. Now, you got to understand that I use a mounting block so the reins are in one hand and the mounting block in the other while we're all circling!! Quite comical at times.