Helvetica, I use the same sort of preparation for scrying that I do for meditation. I usually scry by candlelight. Sometimes I cast a circle, but I always smudge the area with sage. If you are using obsidian, you have some "built in" protection against negative energy. I see scrying as entering the spirit world, so I feel like you need to take whatever means of protection you feel is best.
Get as comfortable as you can. If you are successful (which you might not be at first), you may be sitting for a while.
Look into the surface of the stone lit by the glow of the candles. At first you'll probably just see your own face, but let your eyes "de-focus." (Is that a word?) Don't try to keep from blinking - it's not supposed to be torture.
At first, all I saw was fog, a sort of slow moving mist. After about the third or fourth time, I noticed I saw the fog forming almost immediately, then flashes of images - some were completely unrecognizable as people or things. When an image starts to form, the fog still stays, but seems to cradle the image as the picture emerges.
I will sit until there are no more images, or until the only one that appears fades. Then, still by candlelight, I journal what I've seen and meditate on that. I usually ground myself by eating a few bites of something I've bought into the room with me and afterward is when I snuff the candles and turn on the light. (One of the things I noticed is that if I immediately get up and turn on the light after I'm finished with the stone or other scrying tool, I feel really off-center. It's like I need to gradually return.)
That's just me though. I'm sure others have different methods.