I have been traveling all over the world lately (spending more than half of my time away from home). Since we are still a cost conscious start up, I travel coach most of the time, even for very long flights (Boonty shareholders, take note !).
I am flying Air France most of the time and I have noticed that I am upgraded to business about half the time. At one point I thought I knew how to be upgraded : book a 777 or a 330/340 since I have noticed that about half of those planes are business seats. On the other hand a 747 has very little room for business so upgrades are more difficult. I was also recently upgraded when I strained my ankle and I called my amex insurance, they took care of everything.
So how do they upgrade you ? is it just based on your frequent flying card ? Do you need to be nice to the desk person in charge of check-in ? Do they even upgrade you from time to time so that you are hooked to it (like drug dealers giving away free stuff) ? Do you need to be late at the check in (it happened to me once) ?
if you travel a lot, be sure to check this site : Seatguru.com, lots of useful information for frequent travelers.