Use PGP Encryption
You're being watched. That's all there is to it.
"But I've got nothing to hide!" I hear you saying. Sure, you might have nothing to hide, but if just anyone walked into your home and asked to search everything, would you let them in? If you would, you're more trusting than I am. If that's the case, this article isn't for you.
Otherwise, you may be concerned to know that every packet you send across the Internet is kept by whatever powers are afoot. Be it governments or Google, your information is being collected, and you have to take a few steps if that information is to remain as private as you intended. To ensure that only the intended reader gets the information you're sending, it needs to be encrypted so that only they can decrypt it.
It may seem complicated (and on a technical level, it is), but the process on your end is actually quite simple. There is a standard for this type of public key encryption, and it's called OpenPGP. It's an implementation of technology from the early 90s called Pretty Good Privacy, or PGP.
This encryption protocol is implemented through different software packages. To make things more confusing, the most commonly-used implementation is the GNU Privacy Guard, or GPG. So we have PGP and GPG, which are both pretty much the same thing, but they're different on a more technical level. Since more than one implementation exists, I'll refer to them all as PGP.
You can most easily use PGP encryption through the Enigmail extension for the Thunderbird e-mail client (and derivative works). You can find a quick setup guide here.
Once you're set up, it's fairly easy to obtain the public keys of people you are connected with. I use this pool to upload and retrieve keys.
Here's the result. It turns this message into this message. When the recipient obtains their copy, they can use their private key to decrypt the message and read it like normal, but the middleman gets nothing but garbage to read.
Is it invulnerable? No, but it's a whole lot safer to do this than to send your messages in plain text, practically asking other parties to take a peek. I might also add that you should send as much encrypted e-mail as you can in order to protect yourself. If you only encrypt some of your messages, other parties looking in will place suspicion upon the encrypted e-mails, and they'll target them. Otherwise, they won't know where to start.
If you have any questions, there are lots of places to research the topic, or you can ask me, and I'll do my best to answer.