One other thing I forgot to mention...
When viewing a stereogram, your eyes must be perfectly horizontal to the image. Tilting your head just a couple of degrees can cause the effect to fail.
The easiest way to make sure you have it right is to notice the top edges of the right and left versions of the image in your unfocused vision. One side should not be floating up above the other. They should form one straight edge all the way across.
