Book 9 months ago?
(Though I suspect even in coach, these flights skew more toward closer-in purchases, so a 6-8 week adv purch might put you reasonably high in the priority among other Platinums).
At the end of the day, I think it's a pretty big crap shoot. I've seen, say, the 9am flight at J5 a few days before, while the 8am and 10am flights are J0. Not sure if there are any patterns. And on "normal" travel days, seems that many of the flights are down to J0 or J1 in the last 24 hours, so upgrades are dependent upon no-shows (probably higher than normal on this route).
The advice to choose an inconvenient flight on a Saturday is probably the best advice you can get, though of course seems they've been cutting back the schedule at those "inconvenient" times.