Hub flights in general (but especially fortress hubs like DFW) have a lot of elites competing for upgrades.
You also want to avoid Monday morning and Thursday evening flights, because they are frequented by consultants.
Actually I'm finding now the "golden rules" really don't apply as high end occasional travelers are buying F, particularly when aggressively priced, to avoid the hell that has become coach. As an EXP I'm getting upgraded on what were "heavy" days and totally missing or upgraded at the gate on what were "light" days. Case in point I have a JFK/DCA flight coming up this Saturday PM and right now there's one seat left in F.