Visas
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.
Money
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.
General Tips
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.
Plan big and loose. Read everything you can about the area.
There may be sights and attractions you didn’t know about. A rough outline of your trip might have three or four target points and a variety of ways to get between them.
You don’t want to find out that the weather isn’t what you thought, or the guide book was incorrect, after committing to 6 weeks in a specific spot.
Some trips will allow you more leeway than others. Travel plans in Asia can often be made day-by-day while summer travel in Europe should be organized at least a few weeks ahead, unless you’re prepared to hunt around for hotel rooms and train seats.