Actually, on United the upgrade for non-elite members is confirmed 72 hours before hand. Only on a full Economy fare (Y, YUA, BUA). The upgrades are sold in books of 4 or 8 certificates each good for 500 miles of flight (rounded-up). They cost $200 and $400 respectively.

One other suggestion Delta, I believe, is offering 20,000 miles if you get their Gold Amex Card, this is enough for one roundtrip upgrade any coach fare to next class of service, or two upgrades from full fare coach to next class of service. The annual fee is only about $75 I think. Cheaper than buying the upgrades, and you can confirm the upgrade at the time that you make your reservation, ahead of elite members upgrading the 'hours' before hand.

Good luck!