We usually pay full price on the small things we get like T-shirts, small local crafts, inexpensive momentos, etc. because we feel so fortunate comparing ourselves to the locals that we just don't have the heart to bargain with them. A few dollars mean almost nothing to us, but a whole lot to them...
The one time we did bargain was for a Lladro piece in St. Thomas several years ago. We knew the price in the US, so we were able to comparison shop between a few stores and leveraged one against another.
In general, it's very important that you have a mind set that it's not a battle of wills nor a demonstration of skills. Your goal should be to buy something for a good price, which means you should have sufficient knowledge of what is a good (and fair) price for the item you want to buy. So, do some homework before going. Then, when you get there, be mentally (and emotionally) prepared to walk away without buying anything unless the price drops to the level you think is good (and fair).