Women suck at negotiating. This has been proven time and time again, in study after study. They tend to be afraid to ask for what they want, unsure of their own value and lacking in confidence. I can speak for myself and say that this has definitely been one of my weak points in the past. Business Week magazine recently came out with an article that hypothesizes that this lack of savvy with negotiating is a chief cause in the disparity vs. men and women's salaries.
I can also say from experience that it helps to be aware of this weakness...and challenge yourself to get past it every opportunity you can. I stopped on my way home from work at a printer that was doing my sign for the National Stationery Show. This sign was actually supposed to be ready yesterday, however, it was promised to me by this evening at the latest. Of course, when I arrived to pick it up, it had not been done. After much hemming and hawing and back and forth, I was asked if it would be "ok" if I picked up the sign tomorrow. I calmly looked over and said "it would absolutely be OK...as long as you don't charge me for mounting the sign on the foamcore." Without missing a beat, they agreed and I got $35 knocked off my bill. Granted, this wasn't a huge chunk of money, but I was proud of myself for sensing an opportunity, acting on it, maintaining my cool, and actually getting what I asked for. Wow, is this all I needed to be doing all along?