It seems to me, after reading tons of threads on upgrades, that they are often done for the resort's purposes.....to even out room counts for housekeeping, clear out areas for maintenance, etc. It seems to be completely random.
Short of asking for a free upgrade (which I would never do), I don't think there's any way to increase your chances of getting one.
On occasion, people have been upgraded to concierge. I was, once. But that was the ONLY upgrade of any kind that I've ever gotten in 15+ trips, staying at values, mods, deluxes, and DVCs.
So, the important thing to keep in mind is that upgrades are not nearly as common as they appear to be from reading these boards. People come back from their trips and want to tell everyone about their wonderful upgrades, but no one posts to say 'I got the room category I booked!'. So it gives you sort of a misleading picture.