They don't need/care to. They prefer to simply track the unencrypted (and unencryptable) header information. They can figure out who you're talking to, where you are, where your contacts are, who those contacts are talking to, and how much time lapses between your communication and their replies. They build up a web of information about you and everyone around you: who's influencing who, how important people are, who is a hub, who is a node, who is trivial. A simple 'not-a-warrant' to your ISP gives them your personal info, and all your friends' info is easy to figure out from there. Once they know why you are, and determine you are of interest, they send over a couple of guys with a middle name of "the" to get your passwords using lead-pipe cryptography. They never need the actual contents of the letters.
There are two ways to beat this.
First is to minimize the signal by constantly use throw-away addresses through proxies for both sending and receiving. If all your contacts do the same, trying to track responses and create a network map becomes far more time consuming. However, you have to get these addresses to the contacts, and have a way to synchronize their use, making communication much more cumbersome.
Second is to generate so much noise that any signal is unnoticeable. If every address is sending mail to every other address, there is no pattern to discern. That's right: the best way to fight the NSA is to turn every computer on the internet into a spam server.
