Simple. Know the price a local would pay, then insist on it.
Example: I needed to buy some Baby Alpaca scarves for a friend. After visiting several shops, the lowest price was S/.60 per scarf ($20), but they didn't have the colors I wanted.
Another shop did, but they wanted S/.80. So, I took the scarves I wanted up to the register, pulled out S/.60 per scarf, and made it clear that was all I would pay. They quickly agreed, and we both walked away happy.
They even closed up the shop and walked me several blocks to the central market to buy a cardboard shipping box for $0.25 so I could send the scarves to my friend in the U.S.
Now, I take the time to learn what a local would pay for every product or service, offer only that amount, and assume it is acceptable. I rarely get disappointed.