I did all of that (Mantras of Maharishi Mahesh Yogi Transcendental Meditation, Carlos Castaneda's Astral Projection ++, Silva Mind Control) once in my life and came down with demonic obsessions, it is no joke. The demons do exist, and if you open yourself to them with New Age practices, drugs, homosexuality and other vile sins and a combination of all, your Guardian Angel, your protector from the fallen angels, Demons, is left powerless ( told to stand aside by God ). That is how it works. It is no joke, I have seen many people fall through New Age practices. Lucky for me, I did not really know that I was delving in the demonic when I was practicing those New Age religion methods, and when the Demons started bothering me (at that phase it is called Obsession, not Possession), I instantly turned to Our Lord Jesus Christ and began praying the Our Father for the first time in like 20+ years and it took a few years for God to completely put aside the obsessions. It was the path that God allowed me to go through for my own good and for others in the future, like now I suppose.
Drugs, alcohol, homosexuality and other vile sins or a combination of them is the more common means by which people are possessed by the demons. Normal people have nothing to worry about obsession or possession, because their guardian angel is there to protect them.