I've heard a rumour that on most (American) airliners, you can always ask the stewardess for a free upgrade to businessclass seats (if available) on transatlantic flights, since they don't mind where you sit once you're in the air.
I do believe the "you can ask" part, but I have never seen someone get it. Upgrades are handled by the gate agent. United shows "priority lists" for upgrades for people that pay with miles or upgrade vouchers on many flights.
If you're willing to draw your credit card, paid upgrades are pretty easy to get.