When you're shopping abroad, especially in a place where haggling is common, keep your ears open to see what other tourists are paying before you check out. You might get a better deal if you know what someone else just paid for the same item. Image from gmilldrum.
If you can, get off the beaten path and browse local shops or markets to see what locals are paying for items you want. If you're sticking to touristy areas, or what you're buying is meant for tourists, then keep your eyes and ears open so you know what others are paying. Some vendors may try to charge you more if they think you're from a "rich" country or know you have a favourable conversion rate. Doing a little recon work before you start haggling yourself — provided you're in a shop that haggles over prices, of course — gives you a solid baseline to decide how high you're willing to go.
How to Haggle: Successful Bargaining Tips [Rick Steves]