(From Clare's article archives, Circa 2010-2015)
If your horse wants to get ahead of you when you're leading them, use checks (tugs) with the leadrope, to slow them down, or back them off.
Sometimes it is necessary to halt them completely, to get their attention,
or to get your point across.
If they still want to pull on you, telling them to back up a few steps should do the trick.
Repeat as necessary.
I find a stud chain, used properly, is especially effective with a horse
that pulls on you or rushes ahead of you.
Sometimes, a stud chain is the only thing between a 1000 lbs horse dragging you to a particularly yummy looking patch of grass, and your composed control.