Meeting and hanging out with locals can be one of the true joys of traveling and getting to know a different culture! Don't fret yourself out of having those experiences. Just always keep it in the back of your mind that people's intentions might not always be the best. And also know that you will usually get ripped off at least once in any given country. ;)
Knowing a few of the common ways you might be taken advantage of can help keep your money in your own pocket and also avoid the usual negativity and possible argument that goes hand in hand with this sort of thing. I have experienced every one of these first hand (well, the last one was both my now-ex-husband and my current boyfriend, not me).
Offering to help with your bagAt airports, boat docks, anywhere that tourists might be moving around with their luggage, there will be people offering to help you carry your bag. Most of the time it's not an offer; they just grab your bag and start walking with it. Be polite (in case they are honestly just trying to help) but firm that you do not want them to take your bag and even grab it back from them if you have to. They will insist on money afterward.
I was in such a bad situation where some guys were holding my bag over the edge of the boat threatening to throw it in the water if I didn't pay them.
Walking with you to practice EnglishThere are many legitimate students out there looking for a fluent English speaker to practice their skills with. But there are also those who will use that as an opening to walk with you, then maybe show you the way to wherever you are going (a waterfall, temple, whatever), then ask for money for being your guide.
I usually agree that they can walk with me, but I tell them up front that I don't have any money for a guide. Most of the time they politely bow out. Sometimes they say it's no problem, walk with you, and still ask for money at the end but at least if you make it clear up front you can get them to back off.
Gem scam in BangkokAfter so many years, this successful tactic is still being practiced. I first encountered it in 2000! In short, when you walk up to a popular attraction, someone polite and in uniform will let you know that it's closed for some sort of holiday or another. Eventually they will let you know of special tuk-tuks that will take you on a tour to some temples around the city (possibly even government-sponsored for a free trip). They do take you to some Buddha statues and temples, but they also take you to jewelry shops where they try to get you to buy rather expensive jewelry that you can resell at home for a profit I'm sure you can guess that the jewelry is not something you can sell for a profit.
Taxi priceAlways agree on a taxi price before getting into the car. If you don't do this, the driver will undoubtedly ask for an exorbitant amount of money once you are at your destination. Refusing to pay ends up in a heated and sometimes shouting argument and leaves not only a bad vibe, but a big dent in your budget.
Even metered taxis don't always run the meter. First of all, it's best to know (from a guidebook or by asking someone) what the general price should be. Then either agree that they will run the meter, or agree on a set price.
For public buses and shared vans, try to know the correct fare. Then you simply hand them the correct amount (or as close as you have) as if you ride that route every day. Done with confidence, they will give you the correct change and you'll feel like an accomplished traveler.
Getting sprayed with mustardThere are usually two people in on it together. One person will spill something on you (for me it was having mustard sprayed all over me!) and the other one will try to help clean it up. While they are wiping up with mess with a napkin or whatever, they will be pick-pocketing you at the same time. Nice, huh?
Purse grabThis is the only one I have not fallen victim to (knock on wood). Always keep one hand on your purse while walking around. Purse snatchers will razor-blade your strap and run away with your purse.
In Bali, it's common for thieves on motorbikes to grab purses/bags/phones off of other people riding a motorbike, even if it causes you to fall! Yikes. Keep your bag at your feet or in the seat.