Of course, the drawback to using a resistor is that it will still use as much power to move less air and turn the excess power into heat -- Not a major problem, however.
As you increase resistance in a circuit the amount of current (amps) goes down (I=V/R . . . amps=volts/resistance). The fan itself probably doesn't have a lot of resistance, so electricity runs through it pretty easily, turning the fan blades as it goes through. Add a resistor into the circuit and the electricity doesn't flow through quite so easily, so the fan blades move more slowly and less electricity is used.
The downside is that both the fan and the resistor have to do something with the electricity that flows through. The fan uses the electricity to force air through; the resistor uses the electricity to make heat. So the resistor does waste some of the precious energy, but the total current drawn by the fan with the resistor in-line will go down.