A) increasing the distance between subject and background
B) Widening the aperature (shooting at lower light levels and choosing a a larger f-stop)
C) Increasing the CCD/Film size
D) special lens adaption equipment

This is a common misconception, almost a “rule of thumb”. However, it is not completely without reason. The further away the camera is, the larger the background will appear compared to the forground subject. This means that a small blurry object in the background now becomes a big blurry object when zoomed in. The object becomes more visible, and the actual blur length becomes bigger (since the object itself is bigger), and we perceive it as being softer. But, if you were to then crop out the object and compare it between two shots: one close, and one telephoto, the blur would be identical.

A few years back, when I first heard this “rule of thumb”, I becames suspicious did some basic tests. But I only recently stumbled accross a blow-by-blow article with examples (this is a good read, very well done):

http://www.film-and-video.com/dofmyth.htm

Now, in some ways, this is splitting hairs, because the end result of backing up will be the perception of softness, and for most of us, that’s reason enough to do it. BUT it’s NOT “depth of field” and it doesn’t achieve the same type of separation you would find with a higher DoF.

Also, keep in mind that shooting telephoto means unnaturally flattening the image. Shapes tend to loose their depth, and distances between objects appear to be shortened. This is not to say, “don’t zoom in”, but if you want the forms along your z-axis to stand out, you’d probably want to avoid using super telephoto shooting, and simply move closer.