The things we're fighting can and will possess your ass. You will not be able to do a damn thing about it and I can and will exorcise your ass but it will hurt like a son of a bitch. My suggestion get one of these:
Here's a close up of it:
If you don't want something permanent, draw it on with a sharpie, it'll wear off eventually. At least get yourself a necklace but honestly it doesn't work real good. They can always fling you up against something and make sure it breaks.
This makes it virtually impossible for a demon to possess you. Dean, Elle and I all have them for protection.