There are a few tricks to snap someone temporarily out of possession if you're with them. They are also applicable to oneself in a shutter moment of struggle or clarity. Not every trick works on every possession but possessions lacking any trick seem to be rare. Again, this is not a permanent solution. Might work for days, might work for minutes. It is helpful though.
I'm throwing in whatever I can recall right now. By all means, add to it. I will too.
Snap out:
- Focusing on/being surrounded by flickering light, esp. candles or torches. Additionally, the sound of burning. (Serves to override more intellectual possessions with base instincts; also calming and distracting in its brightness.)
- Listening to the same monotone song over and over again. Also works with nursery rhymes. The tendency to chant those while possessed is actually an immune reaction of some sort in most cases. ("Seems to reduce(?) the energy level of the possession.")
- Very strong, not entirely negative sensory input. The smell and taste of strong garlic if the possessed likes it, for example, or something ultra-sweet. Playing a song they like very loud. While a hug has also worked, this is all kinds of dangerous and stupid.
- Not recognising oneself in the mirror. Will probably happen automatically if the reflection is examined thoroughly enough.
- Many reality checks used for lucidity. (Pinch nose and breathe in, examine hands, try to look at tip of the nose. Always stay critical, though.)
Precautions:
- Avoid eating corpses (aka meat). "This shrinks the potential you give the entity." The more if it's of a killing variety. (Except maybe if it's some kind of plant thing.)
- High salt intake. Don't dehydrate yourself, though. The sheer quantity matters, not its proportion to water.