CloudCoder

I use ExpertFlyer all the time.

Choosing a flight where your upgrade "might" clear is far more "art" than "science".

Obviously, you first want to look at the upgrade space (e.g. 'C'). Those seats are rare birds. Jump on them quickly whenever you find one.

Next, I look for 'I' class. That's deep-discount business class. I figure that AA sells that class only when they need more butts in seats. If AA is willing to sell the biz seats cheaply, it seems reasonable to assume that the flight has sparsely populated. (Note: This is not a hard-and-fast rule, which is why I said "all other factors being equal". For example, the Consultant Army likes to fly late Sunday or early Monday, and late Thursday or early Friday. They often buy at the last minute. So flights during those times will be difficult to upgrade, because the folks who fly All The Time are likely to choose those times.)

I also look at "D" class, which is another discounted Business Class fare. Lots of D fares = better chance of eventual upgrade/ Lots of I fares = an even better chance

Another important factor is Time Of Day. For the LAST flight of the day, AA must accommodate people on that flight, plus all the passengers (from earlier in the day) who bought biz but misconnected. If you book the first flight of the day, there's a smaller chance of losing your upgrade to someone from an earlier flight.