I've used sphere's, cone's, and rocks to put the cloud texture on. If you want some irregular shape, try the rocks. Remember, each time you click on the rock object it provides a different shaps so bring in a bunch and ditch the ones that you dont' like. Experiment with the textures. Cloud textures are fun especially when you tweak it in the texture editor, but I've also found that some non cloud textures work well too. It takes time. No pain no gain.