There's a fine art in shipping things to a cyclist on the road. You guess how long it'll take the package to get to you, guess how long it'll take the person or company to actually ship the box, guess how far you'll ride in that total amount of time, and pick a post office in a small town(with zip code) off your AC map. (Nowadays, of course, you can look the zip up on the web.)
USPS is the way to go. They'll hold the package for you, and if you hit town 30 minutes after they close at noon on a Saturday, pull out a piece of paper, address it to "Postmaster," and ask them politely to ship it the the town you think you'll be in next Tuesday or Wednesday, fold it over, and drop it in the outgoing mail box inside the post office. It's a free service from the Post Office. (So far...)
You can ship to yourself on a one month trip, but if you're going longer than that, you'll need some one to ship to you. Cell phones make it easier to stay in touch and request stuff; before cell phones and the web, AT through hikers had to send a post card from Hot Springs, NC asking friends or family to ship the box to Damascus, VA, where they'd send another post card, and so forth.