Get a decent one with a couple of spare keys and check you can’t open it with a pair of nail scissors jammed in the barrel. These are useful for attaching your luggage to things, or for securing lockers (or even room doors) in hostels and hotels.
Do not get ripped off just because you don’t know the value of the currency. If one person charges you 3000 Dong for a handful of pineapple and another tries to charge you 120,000 Dong for the same stuff, you know you’re being ripped off. But if the first person you encounter tries to charge you 120,000 Dong… that’s when it helps to know that there are 30,000 to the pound. Four quid for a handful of fruit is definitely way off.
Certainly in Asia and Africa this is a must. Everywhere has a local price and a tourist price. Find out what the local price is and start your bidding just below it. Be prepared to accept slightly above it. If the shop keeper won’t go as low as you want, turn round and walk off. Knowing the local language for “too expensive” also helps. If they don’t chase after you with a revised offer, there’s another shop yards away that will. And the guy you’re haggling with knows it.
Even a teeny amount. Enough to say “hello”, “goodbye”, “thank you”, “no thank you”, “go away”, “cheque please” and a handful of numbers. These will help get rid of the people just mentioned and can also make a tremendous difference to prices and service in some shops.
As above if someone asks you how long you’ve been in (wherever you are), tell them “2 months” or “I live here”. Tourists who answer “3 days” are usually easier marks as they’re unfamiliar with the area. It’s amazing how quickly you’re left alone if dodgy people think you’re a local.