Plan ahead for your health–Get your Hepatitis A/B shot, and update your MMR and DPT. Carry Imodium and take it at the first sign of trouble.
Never, ever pass up an opportunity to do the laundry.
And don’t worry about what the cleaning lady thinks re. your underwear set out to dry in the shower.
Know what currency the country you are travelling to uses, and a quick approximation for what that is in the currency you are used to.
Strike an appropriate balance between spending money freely because you’ll never have another chance to do (whatever) and not spending money on things which are too “expensive”.
Strike an appropriate balance between getting the best possible exchange rate for your money, and the amount of time and energy spent obtaining that exchange rate.
In many cases, you can use your standard Visa or MasterCard while in foreign countries, and pay in “the local currency”, and the exchange rate will be handled by the credit card companies.
In many cases, restaurants, shops, taxis, and people on the street who cater to tourists will be willing to accept American Dollars. Remember, this is not a charity, this is a convenience. So, be aware of the exchage rate.
Don’t let worrying over whether you could have gotten a better bargain elsewhere keep you from enjoying yourself.
I think a lot of that is rather over the top. Rubber sandals in the shower? Bottled water for brushing your teeth? Unless you’re REALLY in the back of beyond and purposely roughing it, that’s pretty unnecessary. There are pretty few places, at least in big towns, where a bit of tap water is going to do you any harm. I’ve been to 50-odd countries and I think I’ve drunk the water, or certainly at least brushed my teeth with it, in all of them, and never had a problem. Sometimes it doesn’t taste so great (in fact the worst tasting tap water I’ve come across was pretty close to home, in Ibiza, where it’s incredibly salty).
My own tip, if you’re in a “third world” country - find a** BUSY ** street stall or cheap restaurant and eat there. Preferably one where you can see the food being freshly cooked for you, and it’s clearly sizzling hot. In all my travels, I’ve only once suffered from food poisoning, and that was after eating ice-cream from a five-star hotel bar in Kenya, which had presumably defrosted at some point. (Frequent power cuts + hot climate + ice cream = recipe for trouble.) Local, freshly cooked food is good, pre-cooked “western style” food in quiet restaurants, that is left hanging around for ages, is bad.
To continue good tips if you are in a very warmer country:
stay away from any fresh dairy, saves the day (and the dive; you don’t have to swim around with a cloud of small fish after you)
and, remember that it is often the paper money that acts as a vector, so wash your hands before every meal
Read up on your destination before you go. The plane/bus/train/etc to your destination is a great time to pull out your guidebook and turn to the oft-neglected ‘history’ section. You’ll probably appreciate the place more if you’ve got some context.
Pack the warmest clothes you think you’ll need. Then toss a slightly heavier jacket/sweater/etc in your bag. Even a tropical beach can get a bit chilly at night. Average weather reports are just that - averages. Better to carry an extra sweatshirt with you than pass up on opportunities because you’re freezing.
Everyone, but especially women: Don’t trust guys who want to buy you drinks. Even if you let them pay, go up to the bar with them. The chance of someone slipping something into your drink is minuscule, but still not worth the risk.
This, absolutely. I went to Paris with my sister and, despite her footing the bill for most things, I insisted on actually handling most of the transactions, because she refused to even attempt French. It’s just rude, IMO.
Of course, if you’re not from the US and thus don’t typically carry US dollars, you may have a few problems here. Unless you’re travelling to the US, of course. 
I’ve found that pretty much any hard currency that trades freely on world markets works. I’ve been able to use US dollars, Canadian dollars, UK pounds, Swiss francs, and Australian dollars in a number of places not in those countries. Although as Eureka points out, it is indeeed a convenience, and you should be aware of the exchange. You should also be prepared to pay a reasonable premium for the convenience, if one is demanded.
My tip: Even though airport ATMs and currency exchanges are fairly plentiful, I do not like getting off a plane and wasting time lining up for some local cash. I just want to get going. So I get a small amount of my destination’s cash ahead of time, from a currency exchange at home, before I leave. Not much–it may be no more than the equivalent of $50 or so–but enough that I can buy a quick snack from a street vendor or cafe (or indeed, a vending machine that only accepts local currency), or buy a subway train/bus fare, or pay for any number of other small cash expenses quickly and easily without worrying about or arguing over the currency conversion.
Later, when I need more local cash, I’ll find an ATM or bank in the city itself. And credit cards are great for many larger transactions. But getting some foreign currency before I even leave home allows me to, as I said, get off the plane, get my gear, clear customs, and get on my way as quickly as possible knowing I can cover pretty much any small cash expense that comes up.
Individually packaged moist towelettes.
I can’t overemphasize their importance…
Local beer is better than anything you usually drink. Buy the third round. If you can say “Screw Bush” in the native language, people will hug you and buy you drinks. Never pack any bag that you can’t carry 400 yards at a dead run by yourself. Eyes open, bowels open, mouth shut.
I second even sven’s advice to travel light. Not having tons of stuff to haul around and worry about makes travelling much, much nicer than it would otherwise be.
I am a major fan of the quick-dry stuff from Ex Officio (I’m too lazy to link to it right now.) It’s light, easy to pack, wrinkle resistant, you can wash it in a pail of water, and it dries fast. Not to mention that the stuff puts up with tons and tons of abuse. It’s not cheap to buy, but the money it saves in laundry bills over time more than pays you back.
I also second the tips about eating from busy street vendors and making sure to try the food.
IM not all that considerable E, Spoons’s advice about carrying money is…well…on the money (sorry–it had to be done.) If you’re going to a third-world country, make sure that at least some of the money you carry is in small denominations. Sometimes, people will refuse to sell to you if they can’t make change. That can really, really suck if you want something to eat, need a bus ticket, or want to buy something small in a shop.
Carry your cipro. Get lots of it and bring it with you. Mine saved my sorry butt twice in my last international adventure. I gave some to other people, too, which saved their sorry butts, too. Best thing I brought, after my credit card and passport.
While you’re at it, make sure you get your immunizations and bring any other medication you need (the most current anti-malarial stuff–whatever the local mosquitoes aren’t resistant to yet–comes to mind, here). If there’s any specific personal product you feel you really need, bring it with you. It can suck, spending time trying to find a particular type of conditioner or deodorant or tampon, when you could be out seeing the sights.
If you’re travelling alone, try to stay somewhere with communal breakfasts. You’ll meet interesting people that way.
The two-night rule: if booking a hotel ahead of time, always book for two nights. If it turns out to be a shithole, you usually need a good day to find somewhere else, but the hotel will make you check out at 11 or 12.
Bring earplugs and an eyemask, in case the above is shite - also good for planes.
Don’t bother with a towel: pack a sarong. Lighter, less bulky, dries quicker, doubles up as a bathrobe, a light sheet, or an improvised bag.
I never travel without my shortwave radio - you can download frequencies for Voice of America or the BBC World Service from the internet, print them out. Helps to keep in touch.
Crocs are really good sandals - light, comfy, durable, and don’t seem to smell too bad. (BTW I have never worn rubber shoes in the shower anywhere I’ve been, and my feet are fine.)
If you’re in a place with crazy traffic, such as Vietnam or India, and are unsure of the safest way to cross the street, find an old person and follow them across. My reasoning being that they have managed to get to an advanced age without being run over, so they probably know what they’re doing.
There are three rows at the back of a 747 where the plane becomes too narrow to have three seats at the side. If you don’t mind sitting by the toilet (and you shouldn’t, 'cos then you never need to queue) or getting off last, these are awesome seats to have. The person by the window is essentially sitting on an aisle.
If you’re travelling in a pair on a 747, go for two seats from the four in the centre. Picking a window seat and adjoining seat means you’ll have to ask some stranger every time you get up.
Be nice but assertive. One person in your party is probably better at this than anyone else, so let them do the important negotiations.
Safety is pretty lax outside of the Western world. Don’t assume that there are measures in place to protect you from harm.
Never ever eat in a restaurant that is empty and smells of pee.
It may be empty due to different local eating times, but the pee thing is very important.
These are great, thanks guys. I’ll be leaving my first parent-less, chaparone-less international trip in a few weeks and now I’m all excited.
Here’s my contribution, which I learned from my dad:
You can’t do everything in one trip, so don’t worry if you have a few afternoons where all you want to do is sit in a cafe/park/hotel room and read, or even watch tv… It’s a lot more enjoyable than being exhausted is.
Here’s my best travel tip, international or otherwise. Roll up your underwear and put rubber bands around them individually. It helps you to keep track as your trip progresses, the ones with the rubber bands are the clean ones!
Yeah. Whenever I’ve been to Vietnam, I’ve taken a couple of hundred dollars in US currency, on the advice of Vietnamese people. I don’t think I’ll bother in future, because I’ve never really needed it, and AUD worked just as well - and I don’t get stiffed twice on conversions.
In fact, in Vietnam, take the cleanest, newest banknotes you can find. The state of the physical note itself can have a bearing on how much you’ll get for it (a newer one will last longer, and it’s hard for a Vietnamese local to just go to a bank and exchange it like an American could at home). This gave Australian currency a slight edge, because the notes are polymer and very durable.
Read up on Lonely Planet as to what the local scams are. If I had read my Lonely Planet beforehand, I probably wouldn’t have gotten into a fake taxi at the Caracas airport and robbed at gunpoint (In Caracas, the criminals are in cahoots with the airport security). However in Buenos Aires, when I was squirted with mustard by a passing car, I knew that the women who ran up to me trying to help were pickpockets, thank you very much LP!
My philosophy is exactly the opposite. The first thing off the plane is to go to an ATM and get the equivalent of $300 (less if in a cheap country and planning on a short stay). ATM’s are not always plentiful. I’m sure it’s different now, but when I was in Vietnam, there were only 2 ATM’s in the entire country. In Burma, there were exactly zero. It only takes a couple of minutes and the exchange rates will be the best you will get with only a nominal fee. Then I go to a gift shop and buy a coke or something cheap so that I’ll have smaller bills and change.
I agree with everyone who says “When in Rome, do as the Romans”. Eat the local cuisine, especially from street stalls. Try to respect local norms for modesty (yeah, you damn fat Euros, don’t wear a speedo in Santa Barbara. However, thongs and toplessness are welcome if you satisfy my criteria). If you are prone to a queasy stomach or a vegetarian, you are completely missing out on one of the greatest joys of travel. Drink the local beer and wine. Try the local hard liquor (I’ve got a bottle of pickled snake rice whiskey from Vietnam).
Hide a spare ATM card somewhere in your backpack. Scan a copy of your passport and immunization card and email it to youself on a web based email system (or store in the likes of Yahoo Briefcase). You will not regret this when you lose your passport. Secretly code your important numbers (passport and credit cards) in a fashion only decipherable to you and email it to yourself with a subject line that sounds like spam but is immediately remembered. Preface said email with spam-like glurge to further confuse interlopers.
Some of the most portable practical foods around are peanut butter and olive oil. If you’re an American in Europe, the latter is plentiful and probably much better & cheaper than back home (you’ll have to bring the PB with you). Carry some around with you and buy a loaf of bread when the need arises.
Cheese is also excellent, but spoils much faster, so you have to be a little more careful. I can’t recall many lunches i’ve enjoyed more for less $ than brie and apple slices on a fresh baguette while sitting in a city park in Grenoble.