As you are walking, ask her to slow down for 3-5 steps at a time, reward and repeat. Once she gets it, increase the "slow down" steps to 10-15, then 20-25. Ask with your seat first - sit deep and think "slow walk" - ask with your hands second. Be sure that you ask with a "give take" mentality and not a "pull straight back" mentality.
Circles will help, but make sure they aren't "spins" - purposefully plot and execute a circle, and make sure her shoulders stay under her and not to the outside.
I would advise against stopping/backing to slow one down...they are different cues entirely - and are not really related to a slower walk