Wherever you think you might need it, make your brush smaller and add some touches, like making the smoke dissolve into the air, etc...Its also important that, from time to time you un-zoom a bit and take a good look from a distance. If it doesnt look real, keep on cloning! If you dont want your background to get in the way, you can always insert a completely-black background, which is shown on the picture above. Actually, I was going to leave a black background before I found the dark-room one...