Thanks!
The second best thing to do is to use a e-mail provider that encrypts your e-mail using asymmetric key the moment they receive your mail. It protects you against search warrants but not active eavesdropping of your mail. And if the receiver's mail provider is being eavesdropped or receives a search warrants then there is noting you can do.
Take a look at Enigmail for Thunderbird and Psi for xmpp. Both work quite well and are reasonable to set up and use.
The bigger challenge is secure key exchange. You either need to exchange your public keys in person, or use a difficult to manipulate approach to verifying them, e.g. exchanging keys via unencrypted email and then calling each other to verify each other's key fingerprint over voice communication to ensure the keys were not MITMed in transit.
If I could, I'd encrypt everything, just as a matter of principle. Even if it doesn't technically matter if it gets intercepted, encrypting all your communications is a good way to regain the privacy the government is saying we no longer have any right to. In practice, I'm not going to be able to encrypt everything, because few people are willing to use GPG yet, and that limits my ability to use it as well. But I'll do so when I can, and encourage others to use it.
"We support TLS/SSL with all of our protocols. TLS/SSL is designed to encrypt all traffic and prevents eavesdropping, tampering, and message forgery on any communication between your computer and our servers."