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?

Are you referring to strongbox? Strongbox only works if the institution operating it cannot be compelled to sabotage it.