I used the smudge tool at about 50 percent strength. The brush size varies, I used a 70 pixel brush most of the time for a nice, smooth finish and switched to a thinner brush for the more detailed areas (15px)
Keep in mind that this might not be in relation if you're working on a bigger project or different resolution. Eyeball the size as you work on your dune for the best effect - the bigger the brush, the smoother the sand.
The canvas size I used above was about 400x400 pixels.