I have the New Trent IMP1000 charger which works really well:
http://www.amazon.com/New-Trent-iCru.../dp/B003ZBZ64Q

It is essentially a big battery pack with a USB port. It comes with several adapters including a mini usb (e.g. for cameras), an iPad/iPod connector, etc. A full pack can charge an iPhone multiple times or an iPad2 at least once fully (I have never had the chance to try more). Of note, I don't think it supplies quite as much amperage as an iPad 2 expects and thus sometimes the iPad will report "not charging" but rest assured it is slowly trickling in.

I had an APC battery pack which stopped holding a charge within one year. I've had the New Trent pack for a while now and it hasn't lost any notable charge. Also, when I lost the AC adapter, customer service was willing to send one to me free of charge as a gesture of good will.

Shutting down apps usually will not make a significant difference unless you are running an app that is really doing something in the background like checking the internet or playing audio.

The biggest thing that will improve battery life for most people will be to reduce the brightness of the screen.

Also, on an airplane consider pointing the air conditioner at the iPad. It'll run cooler and might help a little bit. This really helps with laptops by reducing fan usage.

It probably shouldn't matter since your iPad will be in airplane mode, but turning off WiFi, 3G, data, etc can also help a lot, as others have mentioned.