I suppose this could also be applied to photography of other objects, but I prefer clouds
Step 1: Your camera
I use a Canon Digital Ixus 75 for these pictures.
Step 2: The right conditions
Mostly, it is a matter of waiting. Watch the weather and keep an eye out. After storms is a good time for interesting shapes, and was when I took these pictures.
Step 3: The exposure
Setting a high exposure makes the picture bright, low makes it darker (I normally do low-exposure photos - they look quite dramatic)
Normally, you will have to change your camera setting to manual.
In the picture below, the top-left picture is highest exposure, bottom-right, the lowest.
Sorry there are none of my own pictures - I can't exactly take pictures of my camera that well.