This list is in a simple top-down format so that you can download, customize, and print it to your needs.
Things To Consider
Start physical fitness training, hiking, and self-defense.
Find a First Aid and CPR course (American Red Cross) -- three friends of mine have saved others from drowning and heart attacks, two of them were their relatives.
Research destinations including weather, visa, and health issues. Some sources of information: books, tourist office info, encyclopaedia, National Geographic, online archives sites, videos, post to relevant travel sites and newsgroups.
Start immunization shots early.
Get or renew passport.
Get student or teacher status for ISIC card.
Check driver's license expiration dates.
Determine financial strategies: cash, credit cards, traveller's cheques, money market accounts, joint accounts, online bill-paying services, etc.
Stagger annual credit card renewal dates -- this may require a new or different card.
Determine airline ticketing plans -- student discount?
Determine travel insurance needs.
Will you work, volunteer, or study abroad?
What will you be doing when and if you return?
Working -- prepare resume for during and after trip.
Search for overseas jobs.
Ask about a Leave-of-Absence.
How will you file taxes?
Can you pre-pay house bills, home insurance, auto insurance, or will you take addressed envelopes with post-dated checks in your luggage? Can you have them automatically deducted from your bank account?
Prepare cost estimate.
Buy backpacks, boots, sleeping bags & clothes, etc.
Determine where to get visas, and purchase onward tickets along the way?
Get visas. Get extra photos for additional purposes beyond the ordinary entry visa (e.g., in Nepal, each trekking permit requires two photos, and each visa extension requires one, rafting permits also require photos (presumably two). Or if you are getting certified for scuba diving, you will need photos for your application and certification card. Most people think that they need visa photos the size and quality of US Passport photos, but smaller-size photos have always worked (the size of a 35mm contact print proof).
Order airline and rail tickets.
Make hotel reservations.
Get an International Drivers License (stagger issue dates on multiple licenses if going longer than one year).
Join International Youth Hostel Association.
Get permanent email accounts.
Get your own domain of you plan a website.
Get new eyeglasses and sunglasses?
Get a dental checkup.
Don't work until the last day: resign a month or two early if it is a long trip, and try to be out of the house with the car sold one month early. Basically, you will be camping with friends and family, just like you would if you were already on the road, but you still have time to finish many of the critical preparations you weren't able to complete while working.
Prepare a list of important phone numbers and addresses.
Learn about safety & scams, bargaining, customs, traditions & cultural taboos, hitchhiking, telephone usage, postal drops, shipping/mailing, travel writing, and photography.
Do you have a Will? Do you need a Living Will?
Pay the Post Office Box, if you have one.
Make or order business cards.
If resigning, where will they send vacation money, etc..?
Sell/lend/store belongings. (Wash clothes before storing)
Sell house and possessions, rent, sublet, or find a property manager?
Inform homeowners insurance agency, if you plan to rent the house.
Contact Motor Vehicle Bureau if your car registration or drivers' license will expire while you are away. See if you can renew it now, otherwise, fill out a form and attach a check for the renewal, and arrange for this to be paid at the appropriate time.
Contact insurance agents to add renter to insurance policy, or to remove liability and collision if car is to be stored.
Mail forwarding on house or PO Box?
Mail stuff, such as guidebooks, to pick-up points -- GPO or friends.
Bon Voyage Party -- invite your boss!
File taxes or request an extension while on the road.