This afternoon she deliberately signed "milk". I almost missed it. She did it over and over again and looked at me. Then it clicked and I asked her "do you want milk?" and she made her "yes" noise and opened her mouth. I couldn't believe it because she learned "up" just intuitively, but this is the first sign she has learned from our prompts and I only started using it a few days ago.
Repetition is the key. I used it at least three times every time she nursed. I'm so happy I could cry! She is an amazing little girl! I just know once she starts to get that she can tell us things with signs she will be wanting to learn more and more.
Edit: So David came home and also saw her sign "milk". He actually caught her doing it before I did, he pointed and he exclaimed "she wants milk!" He was amazed at how easy it was to interpret.