Practice zazen as if your hair were on fire.
When we sit, we cannot be thinking about yesterday’s actions or tomorrow’s plans. When we while away the time doing this, we are not sitting, but thinking. Sitting should be performed like any other action, we should be consumed by it, so that the thoughts drop away and so the voice of the backseat driver is silenced.
We cannot make this happen. When we try to silence our thoughts, we are thinking. As soon as we realise we are thinking, we are thinking about our thoughts and about not thinking.
When, however, we are completely in the moment, absorbed in the action, our thoughts drop away automatically. If our hair were on fire when we were sitting, we would not be thinking about anything from yesterday or tomorrow, we would be completely in the moment.

Concentrate on balance. There is no perfect balance, we will always sway between too far forward and too far backwards, however slight, and this is how it is supposed to be. There is no perfection. Perfection is only a concept in the mind. If, therefore, we are concentrating on this simple action, our thoughts can be stayed and we can be as free as the mountains.