Discussion with others, drawing pictures, sharing (conferences, email, etc) intuition, basic picture, "how" and "why" in writing, etc, as well as symbols galore.
(Not by a strict textbook read-exercises-read process!)
--Mathematical thinking as a natural human behavior
(The Rassias Method...)
Centrality of (and interrelationships) among expressiveness
(symbolic, written, graphical, oral,...);
understanding; and communication in human learning.