A search will return threads with the excruciating details, but off the top of my head it goes, in tie-breaking order,:
1. UA Elite Status
2. Fare Paid
3. Time Added to Waitlist (which I believe is at the airport when you check in).
Someone noted a lot of NC (upgrade) inventory has opened up recently on TransPacific and Oceania routes, so a call back every so often (with a friendly "Can you please check the inventory?" if they respond with "You're still waitlisted.") might score you a confirmation on your upgrade earlier.