The first thing I did was create what I call a separate "flare image" in Photoshop. I started with a totally black background and made a flare-like selection using the Polygonal Lasso Tool, and filled this area with white, which you can see in the left sample below.
Next I used various blurring filters to soften up the flare, and finally used the Render Flare filter to add some color. The actual technique I use for this blurring and coloring process varies to suit the image I am flaring. finally I added 12% noise to the flare to eliminate the posterizing that occurs in the light to dark transition areas.
The screen shot about shows all the layers used in the process. Notice that the light flare layer was used twice, and that the mode was changed to "Screen". This eliminated the black areas and blurred the whites. In each of these layers I painted in some black on the layer mask to allow more detail from the original image to pass through. Next I added a Warming filter (85) Photo Filter adjustment layer and grouped it only with the light flare layer below it.
Fine tuning of the haze amount was accomplished with a Levels layer you can see above. I added it as the top layer so it would control everything below it. I drag the left slider to the right to bring up detail in the blacks, and stop when I think I have enough. Below is the resulting image.
The whole process sounds more complicated than it actually is. I save the flare images I make because they can be used again and modified to use with future photos.
The original photo was taken with a Nikon D4 and 85mm lens with its aperture set to f/1.6. This enhanced the softness and kept a very shallow focus.