1. Don't die. Don't worry.
2. Walk around! If you see stairs, do climb them. There will either be a tourist attraction, something interesting or a vicious, hungry guard dog wanting to kill you at the end.
Your guidebook will have it's opinion of what it thinks you will find interesting. The tourist agencies will have their opinion. Neither are likely correct. Go wherever you see something you think is interesting. I've had some incredibly boring two hour walks, but I've also had some incredibly interesting ones.
Never in any circumstance bring more money than you can afford to lose anywhere though.
There's no shame in bringing a computer and whatever gadget you feel like bringing. Chilling out on your computer with the wifi from your guesthouse is nice. Talking with your friends back home on facebook is nice. Letting people take part of your life when you're travelling is nice.
There is shame in letting your gadgets rule you though. So just leave your shit in the hotel. Yes, your camera too. Seriously, just enjoy the moment and live the world as it is. The rest of the world will still be there when you get back.
3. Don't make plans! If you meet somebody you like, travel with them. If somebody tells you about this hidden monastery up in the mountains, by all means; go there. If you like a city, spend time there. If you don't, leave it. You're travelling to enjoy life. If you're not enjoying life where you are you're doing something wrong.
Take the bus or the train. Seeing the countryside is good, and you actually feel like you're travelling.
4. Don't give money directly to beggars under any circumstance, this goes especially for cripples and mothers with babies. You really, really do not want to give the local mafia incentives to cripple their beggars, or to steal babies for using and throwing away when they're not cute enough anymore.
It is also quite often really hard to tell who's really a beggar, and who's just supplementing their income by suckering tourists. Appearances deceive, even people who look like cripples sometimes aren't. If you want to help people, give money to the local charity. Buddhist charities are good.
5. Do buy stuff from the poor people selling random stuff (like scissors, nail clippers, combs, socks etc). Pay double what they ask. They are really poor, have shitty lives, but still try to make a decent and fair living for themselves. A lot of them would make more money begging, but they refuse. Reward them for it. 1 US dollar profit from you will make their day. 5 US dollar and they'll invite their friends for coffee, or take a day off to visit their children. Your pocketchange can have an impact on someone's life. Use it wisely.
Do not buy stuff from annoying people who interrupt you or follow you around though. The reason they do this is that it works. Show them otherwise. Buy stuff from people who are not annoying, and make sure the annoying fucker see you're tipping well. If you reward shitty behaviour, the shitty behaviour will continue.
6. Every single person who comes up to you and start talking to you on the street is a liar. Seriously. Normal people don't do this. Their story might sound believable, tragic, funny or whatever, but it's still not true.. they've had a lot of practice telling it.
Buying stuff for them instead of giving money really doesn't work either. The old guy "needing" an inhalator will simply return it to the store as soon as you leave and split his proceedings with his accomplice at the pharmacy. Although everybody is a liar, and it's really important to never forget this, it can still be a bloody lot of fun talking too them.
Be extremely wary of touts and self-styled tourist guides. There's a shitload of shit they can pull on you. The police will just plain never, ever help you either.
One scam is to take you to a coffee place, where the locals will suddenly put on a one-minute show, and you will be hit with a ridicilously high bill for watching the entertainment. Always ask the price for everything in advance. If people get offended by this, it's most likely because they were planning on screwing you over anyways.
7. There's a ridicilous amounts of euphisms for whorehouse and they come in a lot of different shapes and appearances. If there's suddenly seven waitresses smiling at you, standing there waiting for you to finish your coffee you are not in a regular cafe.
8. Taxidrivers at airports are always and without exception the worst scum of the earth. Taxidrivers elsewhere are sometimes scum. Always agree to a price in advance or have them use their meter. From airports, always research how much the fare should be before leaving. Always check the exchange rate before leaving too. Always try to have close to the exact change. If you only have big bills, make them find the change before handing them the money.
Some taxi-drivers are genuinly amazing people with lots of interesting stuff to say though. Sit in the frontseat and talk to them! Be yourself and treat them like actual people and they can be helpfull and entertaining like nothing else. They know their cities well.
A good trick for getting taxis at the airport is heading to arrivals and grabbing a taxi which just let people off. He was planning on returning to the city without passengers anyways, isn't part of the price-fixing mafia and is thus much more inclined to give you a somewhat fair price. Alternativly just take the bus with the locals. It will be cheaper to grab a taxi from wherever you end up anyways.. a lot more fun too.
9. Stay in shitty hotels. Take the local transportation. Eat the local food. Don't do it for the cred, don't do it because "real travellers have to suffer" or any shit like that.
Do it because that's the only way to meet actual locals (most of them are boring as hell, but sometimes you luck out). On the bus to Kashan in Iran, I actually ended up sitting next to a nuclear engineer working at Bushels reactor. He was a genuinly nice person.
Part of travelling is seeing real stuff. Some local guy bringing a prostitute to bang in the hotel next room to you is part of local life. The locals getting shitfaced at the local bar is real too. Staying in shitty places allow you to see the seedy underbelly of life. I find it extremely amusing; even it the most supposedly pious countries people have affairs, flirt and prostitues are readily available. However different we are, we all seem to be the same when you get right down to it.
10. Follow the dress code. If everyone else is wearing trousers, you should too. It's their country, and you bloody well follow their rules for what's acceptable or not. After all, we expect the same when people visit our countries. The less a tourist you look, the less of a tourist-tax you will end up paying too.
When you're tired; get yourself a massage. It's really nice.Sometimes it's amazingly hard to find a massage parlour which offers actual massages though. Do not try getting a massageat one of the massage parlours which doesn't usually offer them (I still have bitemarks).
If you're really tired, treat yourself to a 5 star hotel. In a lot of countries you'll spend the same amount of money for two days in a 5 star hotel and 12 days in a charming guesthouse as you'll spend on 14 days in a mediocre tourist-trap 3 star hotel.
11. Haggling is part of life in most Asia and Africa, and as a traveller you have to learn to live with it. A lot of people love this, but it can also be quite stressfull. Personally, I find it extremely annoying that whether to find out if something is worth buying or not I have to go through 5 minutes of "ooohhh my poor children are dieing from lack of playstation! You have to give me shitloads of money, because that's what tourists do!" to get a price.
Anyways, there's always someone who got a better deal than you did. Don't sweat over it. Do your best, but in the end, the only question which really matters is; "was it worth the price you paid?". If yes, don't worry. If not, learn and do better.
A lot of countries have in separate tourist-prices. With the exception of taxi-drivers (who are scum), I find these prices to be generally fair. It really doesn't matter much to me whether I pay 20 or 50 US cents for my Samosa. It does matter to the guy selling them. If money was that important to me, I could always spend time haggling the shit out of everybody, but I do find that if travelling starts feeling like work I'd be way better off actually finding some work and getting paid.
12. Crossing the road can be amazingly difficult in some countries.
The best trick for crossing the road in pedestrian-hostile countries is finding an old lady or someone with a kid to hide behind. The drivers will be less likely to actually aim for you, as the shit the drivers will get for running down the local almost makes up for the joy they'll feel running down a filthy foreigner.
A lot of countries claim to have the worst traffic in the world. A lot of drivers seem all too eager to do their part to win that competition. Don't die.
13. Always trust your instincts. If something seems like a bad idea, it most likely is.
If people seem genuinly happy to have your business, they will as a general rule not even attempt to screw you over. They will still of course still try to charge you foreigner-prices, but they will honour any agreement and go a bit out of their way to help you. This goes especially for small guesthouses. You can sometimes get amazing service and help if you treat them well.
If the hotel staff clearly think of you as a customer rather than a person, they're way more liable to steal your stuff or screw you over. Externalizing stealing from "some tourist" is a lot easier than externalizing stealing from "that nice Norwegian guy". Be wary.
All in all, I find my first-impressions of hotels and guesthouses to be remarkably accurate in hindsight. Being able to trust your lodging is really important, as they do have spare-keys and you do have your stuff in the hotel. It's remarkably easy to go into someone's room when they're sleeping and take their stuff.
14. Buy a motorcycle. If you like it, buy a license too!