This is highly unexperimented with on my end, so this may only work in some instances.
Before - [link]
After - [link]
DO THIS LAST, AFTER you finish your manip/editing!
Flatten everything and duplicate it.
On the top, duplicated layer: Filter > Blur > Surface Blur
Radius: I usually leave this at 100, but please, experiment first for best results. The higher, the better, I think, because when it's left low, it's so weak that it's used by some to smooth out skin in their photos.
Threshold: This one I leave at 50 - 70 levels. This controls how much fog there is and the thickness of it.
At this point, it's up to you. Play with opacity and maybe even greyscale the duplicated, blurred layer when lowering the opacity.
Now, if any of you keep submitting your render > clouds fog, I'll cut you.
If nothing else, here is another way to add the thick kind of fog: [link]