Usually what happens is that I try to make a circle going to the left and she starts to fall in and then I go "Oh, oh, we need to work on that!" Part of it is just her rightbendedness working against me and the other part is carrying too much weight on the shoulders.
We worked on shoulders Sunday because I had trouble circling to the left; I halt her, put whip into right hand, move the shoulders over (Turn on the haunches), if it doesn't work back-up with whip on the shoulder until she moves. Keep working on it until you've done a complete 360. Then go back to walk work but when she doesn't expect it, ask her to do a square and back-up with whip; rinse and repeat on other side. Then try to circle to the left again and notice that it's suddenly easier.
General suppleness: I have a new favorite exercise which is to bend left for a few steps, bend right for a few steps, go straight, and continue, change rein and repeat. Leg yielding is also a good one or Shoulder In, but those are a little uglier to do with her.