Upgrades are not common, and are purely the luck of the draw. There's really nothing you can do to increase your chances of getting one, as most are done for the resort's operational needs and are assigned before you arrive.
If you ask for an upgrade, the CM will assume you'll be paying for it. If you specify that you want the upgrade without paying, it puts the CM in a terribly awkward position, and I've seen numerous posts from CMs saying that it virtually guarantees that you won't get a complimentary upgrade even if one is available.