Generally device and data protection is achieved by thoughtfully combining three techniques: encryption, obfuscation and deletion. Even in the case of email this triumvirate applies. Encrypt your content with GPG, obfuscate your location with the Tor Browser Bundle, and securely delete the traces left by your regular activities by using a disposable operating system like Tails.
Don’t forget to regularly download, backup and delete your email.
Encrypting your various devices is probably the easiest part. FileVault comes bundled with Macs, and TrueCrypt
TrueCrypt development ended in May, 2014. An audit of the code is underway, however, and community support may continue.
is free and cross-platform. Both Android devices and iPhones have built-in encryption options and can be set up in under five minutes.
Even beyond email, you want to make sure you encrypt and obfuscate sensitive data stored on your computer, in case you are compelled to decrypt it. TrueCrypt is especially good for this; you can use it to encrypt files on your computer and then rename the TrueCrypt file to look like something else entirely, like a movie file. Just make sure that the size of the file makes sense for the “cover” you give it–a 500 MB .doc file is pretty unlikely. While similar options don’t currently exist for most phones, there are research and development efforts in this area.49 If you are traveling, consider whether you need to keep sensitive data on your computer at all. In places with good Internet access, services like Martus can encrypt your files locally and store them remotely; you don’t even have to keep the key with you if you don’t need access to them until you get home. Likewise, if your encrypted files are small enough, you can even transfer them to a USB drive and hide it somewhere that it’s unlikely to be found in a search.
Moving files to the “trash” on your computer–and even “emptying” it–doesn’t actually do anything to destroy those files; it simply lets the computer know that the memory they are using can be overwritten if needed. To really destroy the contents of a file, you need to make sure it’s overwritten, ideally a few times. Tools like CCleaner will let you do exactly that.