Haggling (sometimes called bargaining) is common in some countries, for example Turkey and Egypt. If you don't haggle, it is highly likely that you will get ripped off. Vendors expect a bit of haggling and will state their prices higher than what they'll end up receiving, with almost no exception to this rule. Some points to keep in mind:
A common trick: Shop owners will start with an insanely high price. This will most certainly put you off, but keep to your budget and state your price.
Remember when stating your price, state one that is lower than what you expect to pay in the end.
A good trick on your side is to bid the vendor farewell and start walking off. You will most certainly get at least two offers, each lower than the previous. Alternatively, the vendor may ask "How much do you want this?" (or words to that effect), which acknowledges the fact that they realise a potential sale is walking out of the door.
If there are two or more of you, you can wax theatrical. He wants the item, but she holds the purse strings and won't pay the price, or whatever.
Be strong. Don't let them get to you, no matter how hard they push.
Be courteous and friendly (but firm in your negotiations). If the vendor takes a personal liking to you, you will almost always get a better deal.
You might be offered tea, coffee, snacks etc. You can accept it and it does not mean you have to buy anything. Although you may be 'guilt-tripped' later. Be strong willed.
Do not let unknown locals help you bargain or find what you need. You will end up paying an extra commission.
If bargaining for something unique, don't show too much interest in the item you are actually interested in, or the vendor will know that they're your only choice and price accordingly.
The key to making a good deal is knowing the right price. If you know the right price you can just state you price, start leaving the store and your offer will be accepted. To learn the right price, ask other people what they paid for similar goods and try to make a better deal. If you buy several similar items, try to make a better deal each time.
If you are in a country that use eastern Arabic numbers (0-9=۰,۱,۲,۳,۴,۵,۶,۷,۸,۹) then learn them. It will save you a lot of time and money when you are bargaining about a hotel room and there is a prices list right in front of you. You should still bargain, but it gives you a starting point.
But when bargaining, do so responsibly.
If you make a counteroffer, you're now committed to that price. Don't waste your time or the seller's time bargaining if you have no intention of buying.
Choose your battles. By all means bargain when buying a carpet from a posh bazaar shop. But if a bottle of water is too expensive, buy it somewhere else.
Many items do have a fixed price and only tourist are lured into haggling. For example, groceries and alcohol usually have fixed prices. If you are asked to pay €3 for a bottle of water, do not start haggling, go somewhere else. Do not haggle when buying e.g. bus tickets; check for a pricelist in the bus terminal or ask the other passengers in the line or look over the shoulder of the one in front of you to see what the locals pay.
Do not let the other person "lose face". Often it is said that "everything is negotiable" - but it isn't. Loss of face is never negotiable. Be aware that the person with whom you are dealing has a family and responsibilities. You are trying to find an agreed position.
Find two sellers with the same products and play one off against the other.