Put a dressage whip under your thumbs whilst riding. Have the handle side to the left and balance it across your fists.
Carry the whip across the withers straight. To use the reins you will need to hold the contact with your elbows. Ride all the paces and exercises like this making sure that the whippy end on the right does not travel forward.
The whip acts like a balance bar because keeping your hand steady and forward allows you to keep up with the movement underneath you.
I am off to the Global Dressage Forum in Denmark at the end of the week, and will be back hopefully with lots of news and bright ideas.