As a 1K, this is my upgrade strategy on Polaris-equipped SFO/LAX to IAD/EWR flights:
1. Apply PlusPoints at time of booking, usually 30+ days in advance
2. Periodically check if PZ space is available. Sometimes this happens without the upgrade list being processed.
3. At time of check-in, check for buy up offers. Personally, if $300-400 offer for one-way, I’ll pay it.
4. After check in, every few hours before departure, check for PZ space again. If so, I call in, visit a UA rep at the airport, or talk to the gate agent, depending on time before flight.