Hmm... my way is to use postwork. I put Clay's fire on a sphere because the colors are great for it. I then render, save, and go to Paint Shop Pro (should work in Photoshop too) and use the smudge tool set to about 75% opacity I think and fairly small. I smudge outwards and upwards as you can see in my pic (that's from one of my gallery images). I also added some smoke on another layer; in fact it's probably best if you put the whole thing on a separate layer using masks. Then you brighten it (on suggestion from lordstormdragon, thanks there my friend) and increase the contrast (I used +30% brightness and +56% contrast), and there you have it. I was to lazy to do this last part in my actual image, did it just now for this and it looks 200% better (go look at my original image if you don't believe me). Doesn't look exactly photorealistic but maybe if you played around a bit you could get better results 8-) good luck!