There is a well-known ritual, often played by children, called Bloody Mary. The game is played thus: one or more players go into a darkened room with a mirror, and light a single candle. The main player then stands at the mirror and chants the name "Bloody Mary, Bloody Mary, Bloody Mary" ad infinitum or until wishing to stop the game. It is said that the player will see his or her own reflection morph into that of Bloody Mary, who, if allowed to manifest in full, will then reach out of the mirror and kill the player. If one plays the game with the assistance of a second, it's the second's job to flip on the light switch if things get too scary -- turning the light back on will assuredly send away the spirit in the mirror.
As I was taught, one can also summon other ghosts this same way, simply by chanting their name instead of Bloody Mary.
Now again, this was all taught to me as a kid in the context of being a game to play -- but of course, true believers might feel differently about it.