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, bedding it is clear mind.  When thinking goes, there is only clear mind.  Thinking comes and goes, come and goes.  You must not be attached to the coming and going.

--Seung Sahn