Well, you've all been waiting for this post I know.....MOVED!!
:P
As far as the techniques, it helps alot to study real pics for me(don't worry I'm not saying I'm as good as those other guys), looking at real pics with reflections/shadows/shading helps out alot...also helps to have a little art background, I took art all through highschool(and used to love drawing, until PS came into my life
) and that's it, not much but I really think it helps me out alot in chopping. And of course the other guys already mentioned....PRACTICE.
Another thing that might help you that has came up lately(that I personally have never done).....if you wanna learn how to brush and how to make thing look real, find a part you wanna brush and lower the opacity on it, start a new layer and "brush over" the part trying to get it to look like the real part...