Karma (the manifestations with a volition in actions, words, and mental states) can be good, bad or neutral. For so many past lives, each of us has accumulated multiple vestiges of our own ways of thinking, saying and doing. These make up the individual's propensity or inclination. It is just like a tree leaning toward a certain direction so long that, when it falls, it must fall in that direction. The same with the individual's karma. If we sow good or bad karma seeds day after day, the results we reap must be either good or bad. The corresponding results will definitely appear when there are ripe conditions for them. In other words, our own karma is always there waiting for the right time to emerge. It is not necessarily an instant emergence. It could be a long time (e.g., after so many lives) before the result of a certain karma appears. However, the traces of our volitional actions, words and mental states are always with us, latent but powerful.
Buddhist practitioners who understand the Buddha's teachings about Karma, the interdependent nature of phenomena, and the laws of cause and effect are carefully observant about every action, word and thought(mental state) in their daily life. They train themselves with meditation. Buddhist wholesome meditative practice includes calm abiding (Pāli: samatha), which steadies, composes, unifies and concentrates the mind, and insight (Pāli: vipassanā), which enables one to see, explore and discern "formations" (conditioned phenomena based on the five aggregates). This practice helps transform one's karma for the better. Basically, every second in life, the practitioner must be fully aware of his/her body movements, feelings, mental states, as well as the impermanence, suffering, emptiness, and no-self of all phenomena, within and without.
Buddhist doctrine of Karma emphasizes the important role of the practitioner on the path to Enlightenment. There is no one else but the practitioner who must take the full responsibility to liberate him-/herself from the perpetual cycle of sufferings, and to attain Enlightenment right here and now. Nirvava (freedom from greed, anger, and delusion; or liberation from suffering) and samsara (suffering caused by greed, anger, and delusion) are reality. It is our mind that matters and that transforms it from one state into another.
