One Arm Post Hold
I registered just so I could share my method, which works very well, and can be very comfortable with practice. It is one handed, leaving the other hand to operate the iPad.
Like others, I vary my hold a lot, but I have a unique favorite for lying on my back in bed, head on a pillow, and I call it the one arm post. Essentially, you balance the iPad (in landscape or portrait) in a single hand, with the arm going straight up from your elbow. The grip/hold is the trick, and balance is key, and is done at either bottom corner of the iPad (right side for right hand, left side for left hand). At the bottom corner of the iPad, place your thumb along the front side bottom edge, your index finger along the front side right or left edge (depending on the hand), and your middle and ring finger curled behind the corner of the iPad. Essentially, you are balancing the iPad on your thumb and index finger, with the iPad facing downward, and using the middle and ring finger to control the balance so that you can tilt it for good viewing.
I love this hold. Switch back and forth between hands when you get tired. This may not be best for really long usage, but is great for reading news and surfing the web, etc.