Answer: a bit of thine own tongue
As far as anyone can tell, eye of newt isn't an ingredient in the potion. The other two, while real ingredients, come before the tongue bit. The purpose of the potion is to make known the presence of a person's life force, which can then, being revealed, be bottled, inhaled, or otherwise removed. Thackery Binx's little sister, Emily, was killed when the life was sucked out of her by the Sanderson sisters, at the beginning of the movie.