The sky around clouds is polarized to some degree, most around 90 degrees away from the sun, and least near the sun or opposite the sun. If you use a polarizer, you want to block out the polarized light from the blue sky partially, to increase contrast. You must be careful doing this, however, since [...]

It is recommended to not only take cloud pictures the standard way, with some horizon low at the bottom of the frame and the clouds showing over this. Try other types of composition as well. For instance, zoom in to a small area of a convective cloud like cumulus congestus, to show the convective cells. [...]