One of the most eye-opening and ear-opening things I learned from using my new dual-mirror method of practicing is the observation that I have a tendency to move my bow slightly away from the bridge (towards the fingerboard) when I downshift. This causes a bit of instability, which translates into a less-than-consistent sound.
My solution? Move the bow ever-so slightly towards the bridge during every downward shift.
The results are remarkable! Give it a try if you notice this tendency in your downshifting.