I went to a Kelby Photoshop workshop delivered by David Cross once a few years ago. Among many great tips from Cross was a very cool technique for getting the perfect soft focus for glamour portraits.
In Photoshop, create a new copy layer of the image to be softened. Then right click on that layer and choose "Convert to Smart Object". Then use Gaussian Blur to an amount higher than you would ever normally blur a portrait. Once the blur is applied, double click on the "blending options" icon just under the Gaussian Blue "smart filter" and adjust the slider for the amount of softness you wish. Once that is done, right click the layer and choose "Rasterize Layer". I rasterize the layer as the last step here because if you leave it as a smart object and do any resizing of your image, the blur will change with it. This is why I make a copy of the layer I wish to blur first. I can always go back to the original or even change the blend mode or amount of the new rasterized layer on top of the original. Works like a charm.