Interesting. From a strictly technical perspective (this is impractical), we could set up a warehouse somewhere in the country which accepted incoming mail (tagged with an ID number), dumped the entire package into a plain manilla envelope, then sent it on to the final address.
The final address would be set up via an encrypted web service, so only you and the warehouse know who sent the mail & where the final destination is.
Of course, then the NSA would just take the warehouse's private encryption keys, so it'd only work for about 48 hours, but yeah, you know. Technically possible.
You know, you're not obliged to put a return address on regular mail as long as it has sufficient postage. It's required for various kinds of commercial mail, or mail requiring special handling eg delivery confirmation).
Apparently (https://news.ycombinator.com/item?id=5986011) the letter can still be tracked by looking up the return addresses and point of origin of letters with neighboring unique IDs.
You could send mail to the wrong address intentionally. Like you want to send mail to 3 Main Street, but you write "5 Main Street" instead. The person who receives it will say "Ah, this is for Joe. He lives next door!" and hand deliver it.
That's mostly sarcasm. There's no getting around the tracking if you want to use regular USPS mail or Google Gmail. Just go meet in person, and leave your cell phones at home.
Make sure you walk or go by bicycle or public bus, too. Untracked personal communication and conveyance over long distances is hard in the present system.
By putting another letter inside that may or may not contain another letter ad infinitum, where each letter is sent to a random address except the most inner letter which is sent to the actual address. In each outer letter you put a five dollar bill and the words "Mail this or else, I know where you live!"
This is not an ease of use problem. Using something like textSecure or redPhone is relatively easy and helps greatly with creating a private channel[1]. But there's still a huge marketing problem.
[1]There's still the problems of metadata and backdoors and of legislation around encryption.
The only real solution is to have end-to-end encryption that is easy to understand and use.