"Posted this in another club but want more info and views. How does one become possessed by a spirit? Where does it enter?"
I've been partially possessed on a couple occasions. From what I could see, possessing entities seem to attempt to divide the lower personality from the higher aspects of self (i.e. HGA) and try to become a new HGA for that person. They do this by a wide variety of methods I'm sure, but I haven't seen enough of them to make too many generalizations. From my perspective, at the time I had a rather fragmented personality (divided myself off to protect myself, but never turned into multiple personalities) and a weak connection to the higher aspects of myself at the time. That made it easy for something to just step in and try to lead the lower personality around and almost 'become' it. The first type of entity that did that to me was rather weak though, so it only took me a couple days to figure out what was going on and stop it by just forcing it out of me completely. I think I just had holes in the aura at that time that allowed such things in, due to traumatic experiences when younger that left me psychically damaged. Some people are more susceptible to this problem than others.
"Is there anyway to prevent or fight it? How do we know we are in the danger of being possessed?"
As far as I know, just don't leave open holes in your aura, try not to be weak minded and easily led (i.e. weak in your personal beliefs and with low self-esteem) and stay away from areas where possessing entities might be. It also helps to have a strong connection to the higher more divine aspects of your consciousness and 'The All' (by whatever name you call it). I picked the first one up at a cemetery years ago. Just felt like something came up and slipped into me from out of no where, and then I started having really strange thoughts. Was sitting in a friend's house that night and could feel people breathing in the house and almost felt as though they were batteries to be consumed. That's when I realized there was a problem, and started forcing those feelings out of myself until it finally left entirely.