I just don’t have anything to add to this. Though I’m pretty sure if the NSA put their massive minds to it, they could figure out how to hear us thinking, let alone typity-typing on our computers without an internet in sight. Did you know that NSA is the country’s largest employer of mathematicians? It is. Respect their powers.
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Given that malware can jump the airgap (http://lmgtfy.com/?q=malware+jumps+the+airgap), so it seems you have to overwhelm them with randomly generated noise, encrypt everything, use steganography (but don’t use pictures of kittens, that might go viral), and use something like Tor (but not Tor, as that has problems).
I’ve personally been relying on Venetian Blind signalling, a la The Flintstones. It’s particularly effective with a feather duster.