How to make an iPhone 4 battery last 15 hours?

Going on holiday-12 hour flight and my power button is jammed so I can't turn it on without charging

I don't know about that long, but set the brightness to the lowest visible setting and don't play any games, especially 3D ones. Do simple stuff and it'll last longer.

I'd just try to unjam the button if I were you.

Turn off Wi-Fi, GPS, Bluetooth and put it on Airplane Mode. If you put it on Airplane mode, you will not be able to receive calls or text and you wouldn't be able to send calls or texts either. Turn off sounds and lower your brightness. Avoid background apps by double tapping the home button, holding down on the apps in the task bar and press the red button so that no apps will run in the background.