My mini is stiff turning to the right and doesn't have a great bend. Other than circles and figure 8, are there any specific exercises I can do with him?
Back to long lines, if you're finding the stiffness comes from the shafts. Make sure you have the give in the rib cage without the rig and then build back up. Do you have a coach? I feel like training some shoulder in will help the lateral flexion between the shafts.
I like to long rein a spiral circle. Start with a large circle 20 meters or so and slowly spiral inwards until you reach the center and then spiral back out. I usually get about 6 or so laps around before I hit center. I do this at the end of my session when they are really warmed up and can step under and bend easier. Carrot stretches help a little, too.