So, how do you send something ahead to yourself while you're hiking a wilderness trail and don't know anyone in the next 3 states?
You just send it to the next post office you expect to land at, addressed like this:
Ellie Hamilton (or whoever)
ATTN: Hold for AT Hiker ETA 4/12/2009 (or whenever)
When planning your trip, you estimate your daily mileage and travel time using a trail guidebook or data sheets. The AT Thru-Hiker's Companion notes the number of miles between towns, shelters, water sources (creeks, springs), and other essential information, including landmarks and side trails leading to road crossings or towns, plus a list of post offices. You figure out where you will be and roughly when, and bingo, the Mail Drop.
A Bounce Box is a box of supplies you mail to yourself, pick up at a post office, take out what you need, maybe add something you aren't using much but expect to need later, repack and reseal it, and mail it to the next place you expect to stop. Some hikers pre-pack individual boxes of supplies, maybe a week's worth of food or other stuff, leave them with family or friends along with their list of mail-drops, to be mailed out separately to reach specified post offices by a certain date. There's leeway: A post office will hold mail for a couple months, and then if it's unclaimed, return it to the return address.
Friends and supporters can also send care packages this way. Many hikers post their list of mail drops on their blog or Trail Journal (here's an example.) Whiteblaze.net has an article with more information than I ever dreamed of on how to send a package to a hiker (remember, I'm going to be out there next year....) :-)