I've been able to score a cabana every stay, even when they're "sold out". Aulani doesn't require a deposit for reserving a cabana so I think a lot of people back out when it's time to pay the $450. Here's the strategy I use:
Two Days Before Desired Date: People with reservations are emailed at about 9am Hawaii time to confirm their their reservation. Start calling at about 9:15 and keep calling ever few hours to see if there's a cancellation.
One Day Before Desired Date: Aulani starts calling people at about 3pm to confirm their cabana reservations. Keep calling the reservation line every so often to see if there's been a cancellation.
If you're still not able to secure a reservation, you can line up at Rainbow Reef to see if there are any day-of cancellations. I've always been able to reserve a cabana over the phone so I'm not too sure how that works.
Cabanas are assigned in the order they are reserved. Last minute reservations will likely get cabana 6, 7, or 8.