Set aside all involvements and let the myriad things rest. Zazen is not thinking of good, not thinking of bad. It is not conscious endeavour. It is not introspection. Do not desire to become a buddha; let sitting or lying down drop away. Be moderate in eating and drinking. Be mindful of the passing of time, and engage yourself in zazen as though you are saving your head from fire.
When Master Dogen* (*lived in he 13th century, 51st successor of Buddha, and first true patriarch Zen in Japan) came back to Japan after travelling through China, he made some conferences. In Japan, Buddhism existed already, but Dogen said: "Only zazen* (*za = to sit, zazen = to sit in the posture of Zen) is important. All the Buddhistic decorations, the sutra's, the beliefs, the philosophy,..., all this is secondary. The decorations do not permit us to realize the true awakening of Buddha*..." (*Shakyamuni Buddha is the historical Buddha, but each person who realizes awakening in the posture of zazen, becomes Buddha)