Free upgrades from one resort level to another resort level (ie. being magically whisked away from POP to the GF) are extremely rare and only happen during overbooking.
Free room upgrades from one level (ie. standard to water view etc.) are a little more common, and I think randomly assigned by a computer, esp. if one type of room is sold out.
How do people get them? Luck. But if you really have your heart set on something, you should book it.