It is wise to write on many subjects, to try many themes, that so you may find the right and
inspiring one. Be greedy of occasions to express your thought. Improve the opportunity to draw analogies. There are innumerable avenues to the perception of the truth . . . Who knows what opportunities he may neglect? It is not in vain that the mind turns aside this way or that: follow its leading; apply it whither it inclines to go. Probe the universe in a myriad points. Be avaricious of your impulses. You must try a thousand themes before you find the right one, as nature makes a thousand acorns to get one oak. (September 4, 1851) 

Write often, write upon a thousand themes . . . Those sentences are good and well discharged which are like so many little resiliencies from the spring floor of our life . . . Take as many bounds in a day as possible. Sentences uttered with your back to the wall. (November 12, 1851)

Writing must be exercised and kept strong. 

John A. Murray – about writing as a collective, communal practice:

The writing process is a process by which the individual shows allegiance to that larger community of nature and humankind through labor and sacrifice. The writing process involve the offering up a portion of an individual life to become a part of the cultural DNA by which civilization replicates itself, generation after generation. It is a sacred process, akin to the one at the center of every cell, the miracle by which life does not end after a season or a generation, but continues on through the ages.