Read the room
First you must know where you can and cannot haggle. If you’re in a shop with a proper POS/till, you’re likely not going to get anywhere. In a mall, definitely now. In any shop on the street or a flea market, you’re good to go.
Don’t get too attached
The second first rule of haggling is not to get too attached. You’re almost certain to encounter the exact same thing again, so not buying it right now is not an issue. Plus the sellers can tell how much you want it and will price accordingly. This means keeping the “oohs”, “aahs” and “wows” to a minimum.
This also allows you to walk away which is the most effective Hail Mary in haggling. Sellers know if they don’t close you now, they never will. So, as a last resort when you’re just that final push away from the deal say thanks and start walking away. In many cases you’ll be dragged back and the deal will be done.
It’s all business man
Don’t take it personally. The people selling things are out to make a living and you must respect that. You must understand that they’re there to feed their families and maximize their profits. They aren’t out to get you. Don’t take it personal and very importantly don’t make it personal. Take is as part of the travels and in good faith. You can always just walk away and there’s no harm done.
Your aim also shouldn’t be to “force” the lowest price possible through any means necessary since that would just be a douche move.
Couples have an advantage in nearly all situations, especially multi-lingual ones. When one of you spots something they like the other can play the bad cop and keep complaining how it’s too expensive. If you still aren’t satisfied with the price you can blame it on the other one.
Never buy at the first place you see
Especially at the places that are frequented by tourists — prices there are likely to be double or triple the ones in town. Do shop around and come back to the one that had the best deal or version of the thing you’re after. You also wont be able to gauge the actual worth unless you’ve asked in a couple of different places. If you’re asking for an item for price X and you keep getting turned down you can be pretty certain that you’ll never get it for that amount anywhere and you’ll have to adjust that price.
At places with high traffic you should also lowball hard since they’re probably giving you the highest price they can imagine. Shops near Angkor Wat are about three times the price of Siem Reap, carrying identical things, but they have huge traffic so they can afford to try their luck. If, however, you’re there in a low season, when they give you their price express horror and offer something much lower and you’re likely to meet near the lower end.
Bonus: you probably overpaid, and that’s OK
In many countries people live on a few dollars a day so don’t flash your cash (for safety and douchebaggery reasons) but don’t try to gouge as much as possible. Also know that these people are almost definitely much more skilled then you and you could have gotten a better price but don’t sweat it too much. It’s part of the experience and practice makes perfect.
Me, personally, I always overpay for frigging postcards because I can never remember how much I “normally” pay for them or am in a rush and want them quick. I think every trip I’ve gotten postcards and then seen a place with half the price in 10 minutes, but I am a sucker for kids selling them.