Timefighter, You can also make a new layer, do a cloud filter on it. Reduce the opacity so you can see through to your main picutre. Then what I do is use a lasso, select where I want the dust or smoke to be. Feather the selection like 8 or 10 or however many pixels works for you. Then invert the selection and hit delete to remove all the 'smoke' except for what you selected. Mess around with opacity, and it looks pretty good. Maybe combining that with the others' ideas would look groovy.