A while ago someone on these forums recommended the tutorials and Photoshop brushes of artist/animator Aaron Blaise. Wanting to speed up my workflow, I've since bought a few of his brushes wondering how well they would work in Artrage. I can't be certain if they work 100% as they're designed to, but I'm loving his cloud brushes in particular. They're so easy to use that I actually feel like I'm cheating. Just, sometimes you don't really want to spend an entire day painting a sky. If that's you, I recommend his brushes for use in Artrage! They work a treat.
He releases tutorials on how to use each brush set and I haven't actually watched his tutorial on clouds yet. I'm sure when I get round to it I'll be making better skies than this, which was knocked up in a couple of minutes. From what I've seen on his promo tutorial he makes a lot of use of the dodge and burn features of Photoshop, as well of as the brush-shaped erasers, which are features I'd love to see in Artrage, but you can still make great skies with these brushes in Artrage -- just not necessarily 'photorealistic'.
(His trees brushes are great too, though I mucked around to change the settings of some of them after importing them into Artrage. P.S. I have no affiliation...)