I have several applications where I have done this. Because I needed to read the motor tachometer (which is referenced to the negative terminal), I had to use a high-side drive. If you do it this way...

The circuit used an IRF4905 FET as a switching element. That switches power through a 15uH choke (rated for the full fan current + 25%). The junction of the two is connected to a STPS4130 Schottky diode (both sections in parallel) with the anodes of the Shottkys connected to GND. There are plenty of low ESR capacitors (about 2000uF) on both the input and output sides. Pay special attention to the INPUT side because you will need more capacitance there than on the output side. Put of bunch of .1uF caps there too, to make certain that your buddy's AM radio is usable.
To switch the '4905 gate, I used a Microchip FET driver. The inverting
type is needed, since a HIGH on the output of the PIC must turn on the FET by pulling its gate LOW. A good switching frequency is 78KHz.