Clouds are mostly lucky shots (so endlessly hitting seeds), but you do something to minimize loose flurries by setting the noise variation to a small minus number, say -0.5--1 and experiment with variation and such. You can use the small topview box to see how noise behaves.
I would go about it like this, e.g. If you keep the amplitude small (= simple edges) you can vary the edges by using a surface shader with some degree of breakup (which only breaks up the grey areas).
In this example I've also added a simple shape to contain the cloud, but it's perhaps not needed.