I always end up being tense in the arms and shoulders, especially during the ura movement - where I'm struggling
If you practice suburi with a bokken you'll get rid of the tension. My sensei recommended 400 cuts a days to remove tension. I only manage to do 50-200 every every couple of days. It makes a huge difference.
I recently found that a little kihon gi on the first five principles added hugely to my ability to carry them out. If you practice without the shortcuts of "brute" force to cover flaws in technique, it can enhance your technique a huge amount.