I seem to recall that if you booked a certain class of coach ticket, it gave you a better chance of upgrading.

Was it "q" class, or "y" class...or "z" class? I just can't remember...help!

Also, has anyone actually done it successfully?

Depends on the airline..with AA you can book the cheapest seat and upgrade. With Delta, you have to book their "special" fares

I think it's usually a "Y" fare (at least on American) as my admin assistant always books me on "Y up" fares to give me the best chance of upgrading and it often works.

I think it also depends on whether your travel is domestic or international, whether the cheapest fare is upgradeable or not.

I've been upgraded a few times and I don't think I've ever bought the most expensive fare.

Once was upgraded (I think) because I was Premier status on UA. Another time, we were one of the first to check-in and they were over-booked in coach. Another time was because the airline needed to fill the emergency aisle seat (window) with an English-speaker. I volunteered to give up my seat (aisle) and they put me in first class instead (I assumed as a reward for volunteering). So you never know what's going to give you the best chance of upgrade. If I had to guess, I would say "frequent flyer" with that airline.

Edited: I was referring to "free" upgrades, not the kind you "buy" with miles.

Digging deeper via Google I've found that it is called a "Y-Up" fare. And they are only valid for domestic travel.

For international travel, some airlines offer a "Z" fare, but they are harder to find & book online.