How flexible are your dates? If you have some flexibility, then obviously the best bet is to move your dates around to match whatever upgrade space is currently available. Try to avoid weekends - you may want to go/come back on a weekend but so do most people paying full fare J, thus making upgrade space harder to find.

In terms of 'more likely to have upgrade space', I would vote for JFK then LAX, with DFW and ORD usually more iffy. In my experience, even though DFW has 2 flights, that doesn't help much. JFK is less desirable for many of us, as it usually means in to LGA and then schlepping over to JFK. Do you have particular dates in mind?