For me, I just make lots of eye contact and chat with the CM while checking in (using his/her name) and I always smile alot (these are natural behaviors for me anyway, so I never come across as "forcing" a smile or anything). Then I ask IF there are any upgrades available. Smile. Thank you for looking. Thank you for the upgrade!
Another thing (the thing I think REALLY works for us). When we stay at a moderate (don't do values) I request a building farthest from the main building/food court (everyone wants the opposite). I always get the building I want and have always gotten an upgraded room. Last summer we stayed at POFQ. Asked for either building 1 or 7. Got 7. Then I sweetly asked if there were any upgrades available. Went from parking lot view to river view. AND we were the only ones on that side of the building! So quiet and peaceful and beautiful.
Good luck!