There may be 1001 ways of doing this, but here's my idea:
1- give the "ghost" a transparent look and a slight glow. This will help identify it as the ghost.
2- us mesh warp to give the entering in the body a more fluid look- I'm thinking to keep the ghost in place (no walking forward) and have it flow in the person head first (with some theatrical arms movement) and finish entering somewhat like a smoke tail. Make the movement really fast in the beginning and slower to the end.
3- on the person, draw a mask that expands on the body from the enter point of the ghost, and apply a glow on the edge, to give the impression of the ghost taking over that persons body.
That's it- hopefully you can use all or some of it.
Thanks Ted. I'm going to try your suggestions now :) The effect of the ghost flowing into the body is something I thought about doing, but didn't know how to pull it off. I forgot to mention that in the final product the ghost will be transparent before it enters the body