I ask for an upgrade every time I check in at any Marriott property.. I hand them my Platinum MR card along with the credit card I'm using and I politely say 'If you have any upgrades, that would be great'.. and I try not to sound like Lumberg when I say it.

I find that it works about 50% of the time... I have also been known to call the night before, or en route, to hotels where I have stayed before and they kinda know me, to ask if they have any upgrades available.