The short version of why she signs so well is that she is exposed to a lot of sign, all the time. Her sisters (who are 4 and 9) and I took up signing as a hobby, and got a lot of exposure to sign right after she was born. We invested in a season of Signing Time the Christmas after she was born, and I basically forced the kids to watch it, lol, and started quizzing them on the signs in the videos. Violette loves ST - it really holds her attention.
Then I taught the girls to always sign the signs they knew when they were talking to her. We started slow - more, all done, potty and eat and drink. We've always tried to stay right ahead of her interests, so if we were going to a farm, we'd learn the farm words. Holiday? We'd learn the holiday signs. It has really been a hands on game with me and all 3 of the girls. I'm convinced at some point in their later lives it will come in handy - many schools teach sign as a 2nd language so you don't have to take German or French or Chinese or Spanish. My oldest is taking Spanish at school, so she enjoys the idea that she is learning 3 languages.
We incorporate sign very fluidly in our day to day activity with her. When we are playing, we will sign "more" "wait" or "stop." We sign food words. We sign the ASL signs for the itsy bitsy spider, etc. Our sitter also has watched a lot of hours of ST and willingly has learned and used sign with her as well.
Two things have motivated me. One is that I have no idea how far Vi's spoken language will progress - so basically I've assumed that there is a possibility that sign could be her major communication device and decided to be as serious about signing with her as I would have been if she had been born deaf or hard of hearing. Most kids with Down syndrome do learn to speak - they are just delayed compared to typical kids. If she isn't going to talk for a few years, why not have a way to communicate with us so she keeps trying to communicate.
The other is that I have wanted to learn to sign since I was about 11 years old. I had been on a big Helen Keller kick as a child - she was always so inspirational to me. Now that I have a practical opportunity to learn it, and a good excuse, I've really put my head down to learn sign.
We love Signing Time, and we have really enjoyed this site: http://www.signingsavvy.com/. The older girls ask me all the time "Mommy, what is the sign for X?" and I use signing savvy. They also have a great list of baby signs.
I'm amazed at how well she knows those signs. I'd say she knows at least 75 signs now, and she adds more all the time. I love calling out signs at random that she knows and asking her to sign the words like she did on the video on the blog. Most of the time she can sign them as fast as I can say them. It is really fun now when I show her a new book how she will sign things she sees on the page - I know she is associating the sign with the object.
For us, it really hasn't been a big deal that she isn't speaking more (though she is starting to attempt to say more things, which makes me happy) because she is so communicative through sign. It has been really cool to watch how quickly she picked some of them up from Signing Time.
There are so many advantages to signing as a parent, even with our typical kids. If we are ever in a situation where our little angels are misbehaving, without saying a word to call them out, we can communicate with them.