THE FOUR POWERS OF PURIFICATION
The purification practices found within Buddhism are not unlike the practices applied in many other religions. The most essential mental factor that one requires is sincerity or honesty with oneself. When one wants to purify past negative karma, one has to do some action with the correct motivation.
This is summarised in the following Four Powers of Purification:
Power of the Object: One should practice thinking of all sentient beings one may have hurt. Traditionally, one remembers all sentient beings and the Three Jewels of Refuge (Buddha, Dharma and Sangha), by generating compassion for all sentient beings and taking refuge.
Power of Regret: This should not be senseless guilt or self-recrimination, which are said to be useless emotional torture. What is intended here is to examine oneself and one's actions and to recognise that negative actions done in the past were very unwise.
Power of Promise: As a logical consequence of the above, one should promise not to repeat these negative actions. It is good if one can promise to avoid a negative behaviour for a specific time, or at least promise that one will put effort in avoiding repetition. Not being honest at this stage makes the practice useless or even harmful to oneself.
Power of Practice: Basically any positive action with a good motivation can be used as practice. Traditionally in Buddhism, one can practice e.g. making prostrations (throwing oneself to the floor - as a means to destroy pride), making offerings (to counteract greed), reading Buddhist texts (to counteract ignorance and negative thoughts), reciting mantras etc.
It is often explained that one needs to clear a field by purifying it from rocks and weeds, then planting seeds by study and meditation, giving water and fertiliser by doing positive actions, and automatically new harvest will grow.