Particle System Advice?

[Acies]

Generally:
Set out to make them barely visible. The visibility should be at a level where you'll see them "sub-consiously" or pick them up if looking for detail. If you make them "blam-in-your-face" it will often tend to look worse. Do this through adjusting RGBA values.

Once you've achieved this, go about changeing some of the sizes and heights (Y-axis position of the clouds) until you've gotten some variation into it. Then add some slight color variations to some of them - very slight; it should be barely visible.

Edit: Start looking at the original maps - they'll tell alot about particles, billboards etc.