if you want an easy to follow book that'll teach you a bunch of different techniques try "Down and Dirty Tricks" for whichever version you have, I adore that series for learning adobe software
to change the orientation of the shadow under the trooper in the background use the transform tool. you might also what you change the orientation of the shadows not associated with the one in the foreground...you've got like coming from 2 different directions. Other than that I really like the composition and the sandies really look like they were in the picture, you matched the blur really well