Turn off the radio(s)
If you're not using WiFi or Cellular Data, turn them off. The same thing goes for Bluetooth. Each of those radios consumes power when they're turned on, even if they're not being used.
For WiFi and Cellular Data, the switches to turn them off can be found in their respective Settings panes. To shut down Bluetooth, go to Settings > General > Bluetooth.
Airplane Mode is also useful for conserving battery life. In addition to shutting down WiFi, cellular, and Bluetooth connections, switching to Airplane Mode will also kill GPS and Location Services.
If you still need WiFi or Bluetooth, either of those services can be re-enabled individually while in Airplane Mode.
Keep an eye on your Location Services
Another battery hog is Location services. You can turn off Location Services entirely in Settings > Location Services. For a more granular approach, you can turn Location Services on or off for individual apps.
Turn it off, or down at least
The third-gen iPad and its Retina display suck huge amounts of power. The easiest way to lessen the power draw of your iPad is to turn down the Brightness. Head to Settings > Brightness, and set the slider as low as you can manage.
Unfortunately, on displays like the iPad, it takes as much energy to display a white pixel as it does a black one, so switching to white text on a dark background in iBooks, Instapaper, or your favorite reading app won't help.
Streaming content Netflix, Youtube, or iCloud, is obviously going to tax your battery more than playing local files. The same thing goes for editing files syncing with cloud services like Dropbox. Whenever possible, download your work files to your iPad, then upload edited versions later when power isn't such a precious commodity.