When you do a study, pick an element to change or add. this will force you to have greater understanding of what is going on in the illustration. also, thinking about simplification of values and shapes. try to understand the balance. make marks of intent rather than simply rendering.
cool sketchbook man, the environments are cool, but of course, you need to give them more time, keep working on them, i really like that one on post #57, it came out great! keep working with those values too, overall, great sketchbook man!
Aryeh, I will definitely try to concentrate more on getting the basics of the image correct and also try to really learn something from the studies. Also good tip on changing something in the study. By this I assume you mean the direction of light, new elements etc?
Gpozzo, I am thinking about doing more speed paint enviros just to get a better feel of light, mood and distance. I try to prevent myself from using to much time on the images since I usually end up detailing too much in the beginning even though I should be concentrating on the foundations of the image.