Scrying is the art of seeing shapes and shadows in different mediums and interpreting them. There are many different ways; including mirrors, water, and crystal balls. Scrying is best done in dim lighting, as this aids in the ability to pick out shapes and shadows. Candle light or lamp light is of course best - and by lamps, I mean kerosene lamps.
Let's start with crystal balls. The crystal ball has gotten an interesting reputation over the years, being featured in so many movies as it has. My personal favorite?
Crystal balls are quite possibly the most commonly known form of divination among the masses, with Tarot running a close second. For those who have never used a crystal ball, it can take a while to find one you feel attuned with and longer still to be able to read anything with that ball. The best pieces of glass, crystal or gemstone for scrying this way will be cloudy.
Another popular way to scry is with mirrors, either painted black or left as is. Water is also commonly used. Scrying with water is called Hydromancy. To scry with water, you can use a scrying bowl - a clear glass bowl painted black on the outside. You might even have heard of a famous Hydromancer - Nostradamus.
Scrying can be done with almost any medium which shows shadows and shapes - those mentioned above, as well as fire and smoke. There are many more that I don't feel I have the space here. There are wonderful resources out there. I'm really loving the CrystalLinks Divination Index, which has shell and wind scrying listed as well. Regardless of which method you try, remember that it can take several sessions before you might see anything.
What forms of scrying do you prefer?