floors

It's probably not the subfloor (though you might want to check it), I have a solid cement subfloor and mine creak like a haunted house. It's that the wood pieces shrink over time and the space between the pieces makes them all rub together and creak. Probably a good light sanding, then a new layer of polyurethane will fill in the spaces and take care of it.

I haven't done it yet, I like the sound, myself.

Nycperson,

There is a slight chance that a floor guy can inject a material that can stop the creaking. I have not done this but i hear it is possible

At Robert Deniro's Greenwich Hotel in Tribeca, the woods floors are designed so that they purposefully creak. Here is an archived article from Curbed if you are interested...
http://ny.curbed.com/archives/2007/10/10/deniros_greenwich_hotel_to_squeak_when_walked_upon.php