I feel quite jealous reading some of these stories.
I usually (read almost always) fly economy class and I fly on 20 to 25 flights a year, but I have never been upgraded - not even once.
I thought it was going to happen once. I was flying Cathay to Singapore, and I was (as usual) in the economy queue. People were going through and quickly I was next in line. The attendant scanned by BP to a loud beep and a red flashing light. 'This is it', I thought. She went away and started furiously typing on the computer. Two minutes later she handed me back the same BP.
I suppose I did once get an upgrade. I was flying on easyjet on New Year's Day (yay to working in a foreign country at 7am and January 2nd!), to somewhere in Europe. The flight (as you can imagine) was half empty and nobody had booked and paid for the emergency exit rows. Four of us lucky passengers were moved to those rows with 2 or 3 inches more leg room. The best part actually was having a row to myself.
I shouldn't complain though. I've always got the service I've paid for, and I have had some pretty sweet upgrades at hotels.