I have a horse who has a weak shoulder, and would like to make every circle a diminishing curve. I let her go once, and she just kept going in till she stopped. So I started her on leg-yields until she was pretty good at that.
Then back to the circle: I let her start coming into the circle, then leg-yielded her out. If she went straight, off the circle, that was okay. The point was to get her thinking leg-yield instead of spiraling in, and stretching/strengthening her in a non-confrontational way. Getting her to take the weight on her weak shoulder has not been easy, but the more you do it, the easier it gets.
The hardest part has been patience, and not over-doing it to the point where she could get sore. That would ruin everything!