Great question..... I also had a similar question like this. Perhaps the more experienced can enlighten both of us. Here is a short synopsis of what I have experienced.
In a closet of my house there is an old mirror that my family has had for many years. No one ever puts it on the wall. It needs some repairs on the frame and just looks creepy. It is really old. The glass is primitive, rolled with a wavy texture and bubbles and other imperfections. The reflective surface is painted with silver and is oxidizing in places. My grandmother and my mother told me that it collects things. They used to use it to clean houses that they lived in when they had trouble with spirits. That is all I know about it. I keep it covered and put away because I know nothing of how to handle it.
There is a belief that mirrors can pull in spirits, energy and even ones soul. That is something I personally would be very careful of. Maybe other contributors to the site can give us some idea of how to handle these things. I would like to know how to keep things inside from coming back out!
Mirrors have been used in magic for a long time and like Engwald and others. You can use mirrors for hexing to blessings. Mirrors are capable in the right persons hand to be used in any form of magic; that I know of. They are also used in practical means such as in Feng Shui. I have used mirrors for divination, and contact with other entities.
Some mirrors used in the craft do not have a reflection. They are black and concave. You can make your own black mirror by purchasing a bowl made out of glass or porcelain. I prefer a bowl that does not have a flat surface inside it. Do not buy any bowls with carvings in the inner part. Some people like using a large bowl as it can be easier to use.
If you cannot buy a plain black one. Get some black paint for glass or porcelain depending on your bowl and paint it till it is black and uniformly; to use this new object as a tool in magic. In my studies Myrrh essential oil is my choice when cleansing the mirror as it is believed to protect from those who wish harm in some regions of Greece.
When youre new mirror is ready and it not in your use covers it and do not let anyone touch other than yourself. To charge the mirror use dim light or candle light in a room no sunlight as it can counter the work with the mirror. Uncover the mirror and caress it with love. Give it loving energy and before a few moments kiss it and cover it again. Do not overdo it. Repeat this daily for a week. After this it is ready for magical use such as divination or other types of magic.
The more you work with your mirror the more energy and proficient it will be.
This is all from personal practice and beliefs.
Books of reference: The Aromatherapy Bible by Gill Farrer-Halls
This thread is getting good!
Here is something that may peak your interest..
One of the strengths of the coven is the ability to cast in numerous voices for a single intent. The vibrations of this event send a particularly strong signal. For a solitary practioner, whos voice seems to be a tiny ripple in a stormy sea, the use of mirrors can produce similar results. By placing two equally charged mirrors adjacent to each other and the art of craft between them, one sends an infinite amount of replicated messages, thus amplifying the intention.
Paraphrased from some interesting conversations with multiple sources. This sounds like it could be really effective or really disasterous. The more experienced, please comment.