I recently found a couple pieces of excellent hat drawing reference that I shared with some other artists on Phineas & Ferb, and I thought I'd spread the wealth and share it with everybody else, too.
"Draw the crown as if it fits directly over the skull, not the hair, no matter what angle is used for the hat. Some of the crown will not touch the skull because of the shape of the hat.Center the hat on the face, using as your guide the imaginary line running from the center of the crown down to the chin."
"Study the taper of the crown; Pay close attention to the correct angle of the taper of the crown as this is a style feature...The taper may vary with style changes."
One big reason that "how to draw hats" reference is so helpful is that you just don't see fedora hats very often these days. There's lots of great hat images in classic Bogart movies and other Film Noirs. I just watched The Big Heat a few weeks ago, and I couldn't stop looking at Glenn Ford's grey fedora (well, it was a black and white movie, so everything's grey...).