Clear mind is like the full moon in the sky. Sometimes clouds come and cover it, but the moon is always behind them. Clouds go away, then the moon shines brightly. So don't worry about clear mind: it is always there. When thinking comes, behind it is clear mind. When thinking goes, there is only clear mind. Thinking comes and goes, comes and goes, You must not be attached to the coming or the going.
Zen Master Seung Sahn