A collection of interesting articles I've gathered from different sources...
Shizen (naturalness):
shizenThe goal of shizen is to strike a balance between being "of nature" yet distinct from it -- to be viewed as being without pretense or artifice, while seeming intentional rather than accidental or haphazard. Avoid inflicting man-made elements in your tree. Your Bonsai should be as natural as possible. Artificial looking trees that have been highly refined look silly and serve merely as ‘plastic’ plants.

Incorporate naturally occurring patterns and rhythms into your design