Leave the actual packing until the last minute. You will be a hundred times more efficient with the time constraint.
Except for vaccines. You’ll want to get to the travel clinic at least a month before you go. Your arms may feel like pin cushions but afterwards you’ll be invincible. (Science fact!)
If you’re going to be doing any trekking, it’s a good idea to think about what kinds of activities you can do to train (e.g. short hikes on weekends, running, yoga, going for a walk after work instead of watching Friends reruns and eating stuff covered in cheese). Don’t worry about actually doing any of these things. Everyone knows it’s the thought that counts.
Cancel your cell phone service. When they ask why, feel free to embellish (‘I’ve been chosen to start building a colony on Mars! I’ll put in a good word for you if you stop asking questions and just cancel the damn contract’).
Quit your job. When you leave on your last day, regard your fellow commuters with smug superiority (ha! see you never, suckers!).
Obsessively check flight websites to see if you got the best price. (side note: I found a website where you can track flight prices, and even get a refund with certain airlines if the price drops on a flight you’ve already purchased: Yapta.)
Bring only a small amount of shampoo and conditioner. Unless you’re super picky about what you put on your head (and I am most certainly not; I’ll wash my hair with a bar of soap if I have to), you can pick up whatever you need when you get there. Makes for less weight in your bag.
Actually, that goes for almost everything. You only need to bring the bare minimum. Especially if you’re going to places where the exchange rate is in your favor. In some countries it’s better to dress like a local anyway, and for hot places, the clothes you buy there are made for the climate. Just don’t try to buy a bathing suit in Asia if you are not Asia-sized.
Except for sunscreen… a few places I’ve been to (mostly in South America) have been lacking in the sunscreen department. It can be expensive, and sometimes it can be a pain to find one that doesn’t also purport to whiten your skin.
Make cheat sheets for each country — e.g. currency conversation rates and key phrases in the language — to keep in your wallet or on your phone, for quick reference.
Fret over your bank account. Did you save enough money or are you going to wind up wandering around some crocodile infested backwater, with only six dollars, ravaged by bed bugs (true story!), and far away from your support system? (Look into the Global ATM Alliance. A bunch of international banks have made an agreement where you can use any of their machines with no international fee. For Canadians, this means it’d be prudent to get a Scotiabank account.)
Dream about being lost in airports and forgetting your passport.
After you’ve decided what you’re taking, get rid of the rest of your stuff. This is the best part. You can donate it, sell it, give it away, leave it in boxes in your parents’ basement or a storage container. Whatever you do with it, you’re now unencumbered and free to wander the world.