Here's some tips I made, it is kinda rushed since I'm at work. (You can pass them up of course, I am no pro).
First one: When working values with colours, always try to change the hue when going through lights and shadows (i.e.: Your rock up there, the texture you've done is beatifull, but the colours are a bit flat, when painting it, you could add all the colours, some green reflected from the grass, brown/red from the earth, yellow/blue from the atmosphere and so on). I did some prints and pasted the colours used, yours falls almost on the same range of hue, that happens when you only add white on lights and black on shadows. A good practice for this is a monochromatic piece, try to make a still life painting with all the colours, but the result will have to be a "blue painting". Confusing huh? It works, here's an example : http://3.bp.blogspot.com/_z_-1YfOJXt...0/DSCF7069.JPG . It is mostly a blue painting right? But it has yellows, greens, even a bit of red in it.
Second: Try to change the edges on some of the sides of the object. Lots of shadow = soft edges, lots of light = hard edge. Good link for ya :
Well, I hope I didn't sound too harsh, but I am an art student myself and I know help does wonders.
Hope it helps.