Finances
Make sure that you have enough money for the trip. Create a budget before you go, allowing money for food during town stops (on some trips this will be your major expense), stays at hostels or motels, transportation to and from the traiheads, and extra for emergencies, gear repair, and postage to mail things up the trail, or back home. Don't forget to budget money to call folks at home.
Also, pay all your bills before you go. Estimate your phone bills and other regular payments, and send in that amount before you leave. This is prevent the annoyance of coming home to a pile of late notices and threats to cut off your phone or electricity. Also, pay the rent or mortage ahead of time to avoid major hassles. The last thing you need on returning from a great trip is to fight with a landlord or bank about payments.
Logistics
Stop your mail, newspaper, and other deliveries, and arrange for someone to check on your place while you're gone. This person may need to water plants, mow your lawn, or take care of pets. Provide them with your itinerary, and periodically call them to make sure all is well at home.
Give your trip plan to at least one other person, but remind them that it is flexible. Make an arrangement that if you don't call or return home within a certain period of time, they should notify the authorities to start looking for you. Because search-and-rescue is a big operation and not something you want to frivolously call into action (and because in some areas, people are made to pay the cost), you'll have to make sure the home-base person has a reasonable window of time to wait for your phone call or return before they make that panic call.
Prepare your home for your absence. Lock windows, put valuables in a safe-deposit box or in some other safe location, and ask a trusted neighbor to keep an eye on your place.
If you plan to write postcards along the way, it's easy to prepare peel-and-stick labels on a computer. This will save a lot of time during your resupply visits to town. Stick on a label and a stamp, write a brief message, and drop it in the mail. Your friends will love hearing from you, and you won't have to spend a lot of time in post office lines or copying out addresses by hand.
