I've found I'm most likely to get an upgrade if I'm booked for one night as opposed to, say three nights. With that said I have been upgraded on occasion on multiple night stays.

I also think the hotel uses upgrades strategically from time to time. If they are near sold-out status, it might be easier to sell if they put a Plat in the Presidential Suite and leave the standard room in the inventory, rather than try to sell the Presidential Suite at rack rates to someone off the street at the last minute. The plat is happy, the last minute guest is happy, which makes the property doubly happy, (and richer.)