David Revoy, a professional concept artist, offers up his many insights for your benefit. He does incredible work, and really uses some unique techniques to get the job done. You will learn a ton from the various tutorials he goes through. Many of them are in French, but the pictures are the most important part. There are some in English too, plus some that are all pictures. Programs included are Photoshop, blender, and painter. His tutorial page is here.
