(From Clare's article archives, Circa 2010-2015)
Controlling this particular issue is quite easy, in comparison to some of the others mentioned in the other Leading Techniques articles.
If your horse pushes their shoulder into you when you are leading them,
bring their head and neck towards you, which forces the shoulder away.
Using your elbow in your horse's shoulder is a great way to get them to move away. The closer you get their head to you, the more it sends the shoulder away.