Basically, apply vignetting. I loooove it.

Not only does it increase the saturation of colors around the borders - mid tones are brought to dark, for example - the darkening effect causes the center of the photo to have higher luminance, hence drawing our eyes there.

The final stage would be to apply the film-look border, but hey surely you can see the results from the final pic here:

http://www.furaffinity.net/view/3650181/