If I have to do it in C6 with no postwork, I just use the realistic sky fog settings, along with a couple of strategically placed fog boxes.  But I usually use Paint Shop Pro with the Alien Skin Fluffy White Cloud filter.  I open the render I want to work with, then create a second layer.  In the new layer, I select the entire aree (Make sure the use all layers checkbox is not checked.  Then I use the cloud filter to get the desired result.  I then drop the opacity of the new layer to about 15-20%.  If you look at the newest post to my gallery, you will see what I mean.  I used a soften fil;ter and then the cloud filter.  The effect is very very subtle, which is what I wanted for that render.

http://www.renderosity.com/mod/gallery/index.php?image_id=1721353

You can also make selective layers with just limited areas to get an unaven patchy fog effect.

The main advantage of doing this in postwork, is that you can immediately see what the altered image will look like without having to wait for a test render.

Sparrownightmare

(Typoes included for no additional fee...)