Check with the appropriate consulate or embassy in your country to find out if you will need a visa to visit the country of your destination, especially for an extended period of time. Some countries have extremely detailed and complicated entry/departure laws, and treat visits of a week or two very differently from longer stays.
If you’re traveling to one area, check the cost of living there. If it’s high you’ll probably want to budget more carefully and save some money before leaving. The lower the cost of living the less you’ll have to save, but be sure to have a back up reserve in emergency cases.
Talk to other people who have done a similar trip.
If you don’t know anyone personally, try any of the dozens of online travel web sites full of first-person travel stories covering every possible type of trip.
Set up a pre-trip time-line so you don’t end up with a full to do list your last week of work or school.
Things to consider are doctor’s visits for a check up, inoculations, and prescription refills; purchasing plane tickets; renewing passports and obtaining visas and other documents.
Check your insurance coverage abroad and purchasing additional travel insurance if needed. Don’t forget visiting friends and family members!
The longer the trip, the lighter you should pack. This might seem strange, but it’s true you can afford to lug a heavy bag around for a week or two, but do you want to have anything extra for a year?
Stick to the absolute basics and know what you can and cannot buy at your destination(s). There’s no point in bringing 6 months of toothpaste to Europe or buying a sarong at home to take to the tropics. If you are visiting several climates, try to arrange it so you visit the warmer places first and coldest last. That way you can purchase sweaters and long pants and not have to carry them any more than needed. Alternately, visit cold climates first and then ship unneeded layers home — or sell them off
Make sure everything goes with everything else (if that’s important to you), and remember that layers are always best.