I like mine a bit hotter so I usually go through just a bit of this in the beginning but it's not long before they stand like a rock.
1.) ALWAYS square them up before getting on.
2.) If they move, I'll grab the horn and literally pull them over sideways. You may be a little thing so this may be difficult for some.
3.) If they still decide to move, they will get moved FAST but not in complete circles, only half circles then roll them back the other way, half circle then back the other way, etc. If you've done your ground work they already know how to do this and it's a good chance to just make it better.
4.) Once mounted, Always, I get collection then back a step and swing rt, then back a step then swing left, Always!
They stand like rocks.
5.) Unless your unbreakable, NEVER ever mount while they are moving.
It means they need more work.