Viewing Stereo pairs 
There are two types: convergent and divergent.   In convergent pairs, you view the left image with your right eye and the right images with your left eye, forcing you to cross your eyes.  With divergent pairs, guess what, you view the left with your left eye and the right image with your right eye.  I personally find divergent viewing easier, hence all my pairs are divergent pairs.  If you prefer convergent, download the images and swap them with your image editor.  That is what computers are for.  The following description is taken from a web page byGale Rhodes at the University of Southern Main.  The full page as well as several practice images can be found by following this link. NB: she describes both convergent and divergent pairs.

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How To Do It

To view most printed images, you need to view the left image with the left eye, and the right image with the right eye (called divergent or wall-eye viewing). Here are two methods to try.

Put your nose on the page between the two views. With both eyes open, you will see the two images superimposed, but out of focus, because they are too close to your eyes. Slowly move the paper away from your face, trying to keep the images superimposed until you can focus on them. (Keep the line between image centers parallel to the line between your eyes.) When you can focus, you will see three images. The middle one should exhibit convincing depth. Try to ignore the flat images on either side.

Another method -- the one I used when stereo pairs first appeared in the biochemical literature (can anyone tell me where?): Photocopy a stereo pair from a text or journal article, and cut off the copied page above the pair, leaving the pair at the very top of the page. Then hold the page just below eye level, and look out above it at a distant object. You will notice below your line of sight that the two views are blurred, but superimposed on each other, because your eyes are looking more or less parallel to each other to the distant object. Now move the picture upward into your line of sight, trying to keep the images superimposed, and try to focus on them. As with the first method, you will see three images -- the middle one is three-dimensional.

Here's another technique, suggested by Nicolas Guex.

Tape a divergent stereo pair to a mirror, just below eye level. Then look at your eyes in the mirror above the image. Slowly bend your knees so that your view passes through the stereo pair on the way to looking at your eyes below the image, and then slowly rise again and repeat. At some point, as you cross the pair, the images should fuse, and you should find it easy to focus on the three-dimensional image, because your point of focus for looking at your reflection is not far off.

Finally, try this suggestion from Mitchell Miller of MDL Information Systems.

Use two empty paper-towel or bathroom-tissue rolls like binoculars. Point left roll at left view, and right at right. This should allow comfortable 3D viewing. Then see if you can quickly remove the rolls and keep the views converged.

(In trying for divergent viewing, you may stumble into convergent viewing; after all, it's actually a more natural thing for your eyes to do. With the image above, if you mistakenly view it cross-eyed instead of wall-eyed, it will appear very strange because the depth cues of the image (light, shadow, and hidden surfaces) conflict with those provided by the double image. In addition, the space-filling heme atoms will appear to be inside out, or concave rather than convex. Finally, the histidines (ball and stick) will appear closer to you than the front edge of the heme, while they are actually near the middle of the heme.)
Practice, Practice, Practice

Viewing either convergently or divergently becomes easier with practice. Once your mind sees a pair as a single 3D image, it recalls the experience, and resists your efforts less with each try. I can automatically snap a stereo pair -- convergent or divergent -- into superimposition, and I no longer think about exactly what I am doing. Suddenly, there is a vivid, three-dimensional object floating above the screen or page. I've been doing it since 1969 (that's a hint about the source of the first biochemical stereos!), and I've suffered no damage to my vision. It's very handy to be able to use the stereo pairs in Science and other journals without digging up a viewer. Such important tasks as interpreting electron-density maps from x-ray crystallography are practically impossible without this skill.