ReverseEMF:
Here's another way to do it (possibly a little tricker for the nubie):
That circuit can't work for a power MOSFET which has a body diode, since that would simply conduct all the time. A signal MOSFET (no body diode) won't have a low enough Rds(on) for most cases I fear.
As stated previously normally you need 2 active devices to perform this task.