The least amount of leaning or twisting our torso tremendously decreases our stability. Keeping vertical and straight is extremely important no matter what we are doing, meditating, walking or fighting (from a standing position).
A common advice given to help us keep this proper posture is to imagine a golden thread glued to the crown of our head is pulling it upward.
Now it is a beautiful image, but it is not easy for everyone to imagine this kind of things.
Last November, my Sword Master Carl Long Hanshi told us to make sure to always see our belly or chest in our peripheral vision when looking horizontally.
I like this much better than the Golden Thread way, and it also works for Martial Arts as well as for Meditation. It is easier for most of us to see things than to imagine them.
Positive secondary effect : Deeper breathing.
Additionally, while this should be part of our formal Zendo and Dojo practice, it can and therefore should also be practiced in everyday's life.
This is primordial.