It is a very important principle to let the organ of thinking work in yourselves. You are practicing this best of all if you try not to think for a while, howsoever short.
A big, immense decision belongs to it to sit or to lie somewhere without letting thoughts go through the head. It is much easier to let your thoughts surge up and down in yourselves, until you are released from them by a good sleep than to tell yourselves: now you are awake and, nevertheless, you do not think, but you think nothing at all. If you are able to sit or to lie quietly and to think nothing with full consciousness, then the organ of thinking works in such a way that it gains strength in itself, accumulates strength.
Who puts himself in the situation over and over again not to think with full consciousness notices that the clearness of his thinking increases, that in particular repartee grows because he does not only leave his apparatus of thinking to itself by sleep, but that he lets this apparatus of thinking itself work under his guidance.
Source: Rudolf Steiner – GA 57 – WHERE AND HOW DOES ONE FIND THE SPIRIT? – X. The Practical Development of Thinking – Berlin, 11 February 1909