Erwt didn't mention this but I find this quite useful: flipping your canvas occasionally.
I find it most useful when painting characters but this trick applies to "landscapes" aswell. Gives a VERY fresh perspective on what you've been working on. Flaws pop out instantly (in a good way).
Keep up the great job! :D
And that was a very interesting read erwt. I saw those pics on your blog and was glad to know your process - very concept-artist-like! :)