What can one do to reduce the risk of spirit possession (ghost possession)?

Following are a few things one can do to reduce the risk of being possessed.

At a spiritual level:

Do spiritual practice regularly: Spiritual practice that is done regularly and which is according to the six basic principles ensures growth in our spiritual level. As a general rule, a person is impervious to attacks by ghosts at a spiritual level 10% less than that of the person. Continually and progressively increasing spiritual practice and a resultant increase in spiritual level is the only sure way to get sustainable and progressively higher protection from negative elements in the spiritual realm.
Application of spiritual healing remedies: Regular use of spiritual healing remedies described on our spiritual healing page as a preventive tool also protects us from being possessed by ghosts.

Reducing personality defects: Personality defects such as anger, greed are like open sores on our mental body. Ghosts mostly use these personality defects as entry points to gain control over us. We have provided information on how to remove personality defects in our section on Personality Defect Removal. One of the advantages of improving our spiritual level is that there is a reduction of these defects as a by-product of our improved spiritual level. However when we proactively work at reducing our defects we close the window of opportunity that ghosts have in taking control over us.

Sāttvik activities: By making our life and our activities more sattvik, we reduce the risk of ghosts possessing us. As ghosts are predominantly Raja-Tama, they do not like a sattvik environment and hence are less likely to possess a person with a sattvik life style.