Muslin effect

Prepare the image as you would normally, adjusting your levels and Hue/saturation (other filters) to give a well balanced shot.
Duplicate background layer (keep adjacent to background) and add 30 pixels of Gaussian blur. This seems excessive, but ultimately it is the light level radius from a point we are looking for. The greater the blur the greater the appearance of a glow there will be. Now change the blending to lighten. Adjust the opacity to give the desired level of softness and glow. If the softness is now high enough then you will need to increase the Gaussian blur in the duplicated layer (ie start again).

The diffuse glow and a blur can create a similar effect, but is less controllable and requires more fiddling around, and often does not provide the soft glow I am looking for.