The filters and such in Photoshop are nice, but they can be a tad cliche. I would suggest learning the different tools for drawing (pencil, brush, and airbrush), how to make and mix colors, et cetera. Looking at a couple of books on oil painting and watercolor doesn't hurt, a lot of the old school art techniques apply well in Photoshop. Actually, Photoshop taught me how to work with oil paints. Not that I am Monet or anything, but it is really a fantastic program.