It seems obvious when you think about it, but I don't hear it said very often: The first step toward getting what you want is knowing what you want. You have to do more than recognize what you want, you have to be confident in it.
Here's an example from my college days: I worked at a bar for a couple of years. On weekends, we'd have someone at the door checking ID. There were plenty of people who tried to talk their way in without any.
One night the regular doorman was late, and I covered for him. The first girl who showed up without ID smiled, batted her eyelashes and asked "pretty please?" I said something like, "Gee, I really wish I could, but rules are rules, etc." She argued, I refused, she got mad ... I'd like to chalk my behavior up to my youth and her smile, but the bottom line is I didn't project confidence in what I wanted.
The next girl who showed up without ID (for some reason guys never tried this with me, I don't know why) I just told her I needed ID or she couldn't get in, sorry. She threw a quick pout, then left to find someplace else she could get into.
As I thought about this later I realized the first girl wasn't mad because she wasn't allowed in. She was mad because I gave the impression I might be open to negotiation but I wasn't.
Moral of the story: Know which of your goals are negotiable and which ones aren't. Never give the impression that one of the former might be one of the latter. Reasonable people can respect an honest difference of opinion, but will read indecisiveness as an opportunity to negotiate.
Update: I found another way of putting this, on Scott Berkun's site where he explains why things suck:
He was pointing out that people don't complain about things they don't care about, but it works really well to explain what happened to me when I was working at the bar.
It's the things that tease us, making us think they'll satisfy us but then failing, that hurt the most.