The key to soft focus, I think, is to be able to control it by combining soft and sharp. The classic soft focus lenses that I like, like the Verito, Heliar, Pinkham-Smith, and the new Cooke PS945 (I own the first two, not the latter two), do mainly this by allowing the photographer to control the amount of spherical aberration, so a diffuse image formed by the rays that enter the the lens at the edges is superimposed on a sharp image formed by the rays that enter the lens at the center (depending, of course, on how you focus the lens). By stopping down the lens (or adjusting the element spacing with some lenses), you can change the proportion of diffuse to sharp.
Soft lighting and a soft lens often doesn't work. With the Imagon, for instance, the manufacturer recommends a lighting ratio of about 1:5, which would be way too contrasty for a normal portrait (1:2 or 1:3 being the usual range).
I've tried some B&W landscapes with the Verito, and there I find I usually need a red filter to balance the soft effect of the lens, which I almost never use with conventional lenses, because the black skies are usually a little too Wagnerian.
So if you're not using a classic lens, there are other ways to do this. For instance, if you get diffusion with a diffusion screen in front of the camera lens, you could make a double exposure with the diffuser and without, or you could use harder light, or contrastier or more saturated film (or up the contrast or saturation in PhotoShop). Or if you diffuse in PhotoShop, you could have a sharp layer and a diffuse layer and adjust the proportions to taste. I usually find diffusion under the enlarging lens a bit unnatural (recognizing that sometimes surreal or ghoulish is desirable), because it sprays the shadows into the highlights (with negative processes, as opposed to Ilfochrome), rather than vice versa, but when I've seen it done successfully for a natural effect, it usually involves a very weak filter, like a Softar I, used for only part of the exposure time. These effects won't be the same as with a classic soft focus lens, but it still might be a nice effect.