In the Drop Shadow section (above) you can alter all the variables of the shadow, i think these are pretty straight forward, the best thing to do if in doubt is alter the settings and watch to see what happens to the image (make sure preview is ticked) nothing is approved until you click on the OK button. I never use any rules, I just alter the settings and look to see how the image looks, and when its looking good i press OK and save it (great believer in messing about to see what happens).

And heres the shadowed screw head using the above settings.

Click here to see how to add a cracked effect