...and UA doesn't always fly equipment with large F cabins on this route which makes it even harder. The lie-flat 752 is nearly impossible so I try to book the sCO753 for the best chance. I am probably <25% on that at non-peak times as a 1K. Your best bet is to look for a TOD buy up if F is important to you.

If you don't see R space and wish to take the W-fare roulette chance of waitlisting - book a larger plane with more J seats like 777 or 747 (avoid 787, 767, 757's). The direct flights from West coast are generally harder to upgrade, but when booking far in advance I have had them clear off the waitlist. Flights from IAD or EWR [I]might[I] have a better chance given their shorter distance but the actual chance will vary by date and route.

Best bet is to find R space available at time of booking. Remember to search segment by segment only, with most desired TATL's first as R space will only show availability for all segements combined for connecting flights (R=0 even if one of the segments has R=1+ if any segment is R=0). Don't expect domestic hub to hub R space to open up at any point prior to the gate at T-1.