The key to earthbending is through the mastery of jings. In the episode "Return To Omashu", Bumi told Aang that the concept to earthbending are jings. The jings introduced in the episode are the positive, negative and neutral jings. The positive jing is fighting, the negative jing is evading and neutral jing is waiting for the right moment to strike.
Another important key to earthbending is a bender's stance. In the episode "Bitter Work", Toph tells Aang that in order to move earth, he must be able to face it head on, like a rock, as well as be immovable. The only way to move a stubborn element is to be just as stubborn.