I almost always get upgraded as I'm Presidents Circle. The 2 best times this happend where as follows
1) Dubai Airport
Booked a C or D I think. Turned up and asked if they had anything I could upgrade to. They politely said there was nothing available. Then I pulled out my Presidents Circle card, and the guy was straight on the phone to the depot telling them "I have a presidents circle here now - you need to bring me something". 15 minutes later theres a Lexus LS330 waiting for me outside - very nice!
2) Munich Airport. I booked a convertible BMW 3 Series.
Turned up and asked if they had anything to upgrade me to. They told me they did, but it wouldn't be a convertible. I explained I was driving around Europe (All the way to Spain) and wanted a convertible. Then they realised the car the BMW they had assigned to me was due for service anyway so guess what, they gave me a convertible Jag XKR - and yes guys it is true that on some of the German highways you have no speed restrictions! probably the best trip of my life I put 8000 km onto that car in 10 days!
The moral of the story if there is one is that in both cases - and others where I get amazing upgrades, I go when they aren't too busy - find someone who looks friendly and then just start chatting with them. A little bit of good rapport goes a lot further than demanding you get an upgrade which usually fails!