Milk Production decreasing in nursing mother

Please help me!

I had a beautiful, healthy baby girl on June 11, 2006. She was a great nurser from day one and I have never given her any formula, but lately I just feel like my mild supply has been drastically decreasing and I am worried about her gaining weight and also being dehydrated as it seems like her top fontanelle is really sunken in. She is almost nine months old and I nurse her about 4-5 times a day (she sleeps all night), but it just doesn't seem like she's getting much from me. I would really like to nurse her and not give her formula if at all possible. I am eating well, but I have always been really thin. I am trying to drink a lot of water, and it's not like I am running miles every day, though I do try to go for a walk in order to exercise. I would really like to nurse her until she is a year old, but I am really worried about making it to that point. What can I do to increase my milk supply? I have tried pumping, but the pump I had just didn't work very well at all. My left breast is almost completely empty, and my right side is a little better than that. Is there anything I can do? Is my baby getting enough milk? She seems happy and isn't sick hardly ever.

Thank you!

Caysja's Mom