Don't you just hate it when you're in a hurry and your phone is about to die? Yes its a first world problem, but we share your pain. There's no silver bullet to charging up your iPhone but if you follow these few hints and tricks, you can definitely charge up your iPhone much faster!
See also: How To Transfer Charge From One Phone To Another
Put your iPhone in Airplane mode. That way, your iPhone won't be wasting energy on notifications, text or searching for a signal. Alternatively, just turn your iPhone off altogether so it doesn't use any juice at all.
Remove your iPhone's case. Batteries don't like heat and don't respond well to it. In fact heat can actually significantly reduce a battery's ability to hold charge and your actual case could be generating excess heat which is not good for the battery. So try to keep your iPhone at or near room temperature.
This is best one. Use an iPad charger instead of the one which came with your iPhone, it's a larger adapter that provides more juice to the battery.
If you're like me, you probably can't leave your phone alone for two minutes, but resist the urge. When the display is on, it will drain your battery.
Always use a wall charger instead of a USB port as it will charge faster. Apple's official advice is "for the quickest charge" is to "connect the device to a power outlet using the USB cable that came with the device and an Apple USB power adapter."
By HereToHelp (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons