Unfortunately, I think "haggling" is a bit of a necessity for both parties. Not that its lovely or anything. Its just that, unless both parties somehow have the same price in mind that makes them both supremely content, nothing would ever get sold without "haggling". Actually, I prefer to call it "negotiating".
I just have an unspoken rule that I like to give and receive: I don't try to rip anybody off and know that things cost money. As long as I get the same back, whether I'm buying or selling, everything is cool. This is when dealing with private parties.
My tip for negotiating is to be informed. Know how much things cost and know what they are worth. Know the average store price, know the online auction price, know the used price, know all prices. This way, you know whether you are getting a good deal or not. If its a good deal, to me it is not worth negotiating for a couple measly bucks here and there. It'll average out at the end with a smooth transaction and a happy buyer/seller relationship. If its overpriced, be honest with the seller, tell them it is a little more than what you can buy it for elsewhere and if they can do a bit better, than they have a deal. If they can't, say you understand, good luck to them, and move on. If it is a buyer negotiating with you, tell them your lowest price and if they still can't meet it, tell them that you can't do that and thank them for their time. No funky games or trickery...just a straight up discussion of what both of you can afford to buy and sell something for.
When it comes to retailers, especially the big company ones, the gloves come off a little. Why? Because they can either sell to me or don't. If they sell it to me at a price I ask for, I am making the assumption they are still making money for it as their business is to make money, not donate to my favorite charity: ME. I tend to not try to bottom out independent store owners as they work on tighter margins. They still are a business so if they sell to me I still assume they are making money and they don't have to sell to me, but I understand they have to make money like I have to make money. And the extra couple bucks here and there goes a long way for them as it does the customer relationship, unlike most big-chain stores.
I think overall that unless you do want to resort to tricks or deception, a poker face is not needed. Sure, some retailers will not budge on an unreasonable price if you seem like the eager type, but all it takes is your honest description of your budget. If they still feel like its worth playing the sucker game, then find another way to buy the item. I assure you that if there is room to move, he/she will literally call you back into the store to suddenly reveal that the manager has approved a discount!
It's all about a sale to these stores, if they know your a buyer they will come down and give you the deals within reason you will want.
