While I was abroad traveling in Asia I went to many open air markets like the Chatuchak Weekend Market in Bangkok. You're gonna have to do some haggling whether you like it or not, unless you don't mind getting ripped off. Prices are often marked much higher than the merchant is willing to sell it for. Here are some tips I learned. I don't remember where I learned them all from, probably from reading websites, books, talking to other travelers, and just being there to experience it all myself.
1. Do not haggle on prices for food. This is not acceptable.
2. Put your money in different pockets. Split it up, you'll see why later.
3. Scout the market out. I like to walk around the market first and check out the prices. If there's something I want I try to keep an eye out for it and note the price everywhere I see it. If you need to, find a safe place (nobody looking) and move the amount of money in your pockets so that 1 pocket has the amount you want to pay. Put some more in another pocket just in case.
4. After you have an idea for the price find a merchant you want to buy it from.
5. You don't have to be nice, but that doesn't mean you have to be a jerk. Start low, so if you wish to pay 50% of the listed price start at 25% or 30%.
6. Always smile while negotiating price. You can politely point out things that are wrong with it and say it's not worth that much -- WITH A SMILE. Most of what I've read from hippies say don't do that because the merchant will lose face. I say hogwash, I've never seen one care -- they just want your money and they know they'll never see you again.
7. Invariably you will probably lose this battle of haggling. They do this everyday, and you are used to going to the Mall and just buying at the window price.
8. Ok this is where the real trick starts. Remember the money you put in different pockets? This is where it comes in. If the merchant isn't going for your price put your hand in your pocket and pull out your money. Hold it tight and wave it in front of them and say the amount and "I buy". For example, "30 baht, I buy!". Often seeing the money makes their eyes open wide and they'll go for it. Congrats, you have successfully haggled for something. If they're still not going for it, then you have a stingy merchant or you are just too cheap. Move on to another merchant and try your luck again.