Knowing what offers you are willing to accept (where "offer" includes all parts of the deal as appropriate--price/salary, working conditions, options, perks, make, model, color. But keep in mind that this is seldom a single-variable parameter; many negotiators will try to give a customer what they want but GetThemOnTheTradeIn). But NeverStateYourNumber.
Knowing how much time and effort you are willing to spend on negotiations.
I told a friend that I had a hard time negotiating in e.g. Mexican marketplaces, as do many Americans, in part because I had no idea what my true maximum prices was; I just didn't want to screw up.He pointed out that in this case, it wasn't that I truly wanted to negotiate well (which would imply that I did know some non-ridiculous maximum price), what I wanted was the experience of negotiating well, which might be a very different matter.He's right. If the guy selling me the turquoise ring made me feel like I was the world's best negotiator, I might well pay twice what he usually charges! (This all assumes that everything is relatively affordable to my budget even at twice the price, which is why I don't have a true max figure in mind.)
Actually, this is what all of them are doing most of the time, so it is not really "twice what he usually charges" but simply as much as he usually charges. :-)
In my experience, I observed something quite different from the one-upsmanship and ego satisfaction from sticking it to the customer or the seller, it is a matter of making a sale, any sale, which makes the seller a profit, and satisfies the buyer. It should be a matter of utility and affordability, not just in making a good deal. Is the article really wanted and is it a UsefulUsableUsed artifact? Do I really need it, and what is it worth to me? (This assumes a purchase for use, and not a purchase for resale or accumulation).