How to see
To see the 3-D effect, you should try to diverge your eyes more than what you normally do. When we look at near objects, we tend to converge our eyes so that both of them see more or less the same things. However, when we look at far objects, both eyes would be looking in the same direction (this is what I call diverging the eyes). Therefore, the trick is to relax and imagine that you're looking at a far object when viewing a stereogram.
(1) Move your eyes close to the stereogram so that you can't focus on it at all. Relax your eyes, don't try to focus.
(2) Remember the relaxed feeling.
(3) Now, move the stereogram away from your eyes slowly and maintain the position of your eyes.
(4) Move until you see the 3-D effect. If you can see the stereogram clearly and there is no 3-D effect, try the whole process again.
Most people require some time to master the technique. So keep trying until you succeed! :)