The traditional way is mixmaster remailing - mail a letter containing a letter, with instructions to remail the innermost letter, in blind cover. It's a physical instantiation of the way Tor works, and the way early email mixmasters worked. You can use additional 'hops' if you are worried about it.

http://en.wikipedia.org/wiki/Mixmaster_anonymous_remailer

That said, the problem is that most of the time the data you're sending outs you in some way.

Wasn't something like that published recently?