7-9 months is perfect, but we started at birth just to give ourselves more practice with doing the sign and saying the word at the same time. It may be some months before your daughter will sign back, but she will be taking it in. I did a lot of research beforehand and I totally disagree with Amy from Las Vegas. The research shows, in fact, that it actually jump starts their language development because they are using language (i.e. a concept goes with a sign that is meant to represent it) long before they develop the physical ability to speak. Our now 3-year-old is highly, highly verbal and his language skills are incredible. He is starting to read and can write all but a few of the letters. I highly doubt he is going to have trouble in school or end up in speech therapy. I give a lot of credit for this to sign language, but we also talk a lot to him (from birth we basically have narrated EVERYTHING we do) and I think this really is the most important piece in language development.
We also felt that it was very important that we teach him ASL signs rather than "baby sign" that is more made up, because our hope is that he continues to learn more and more signs and can turn this into a second language. So, we used the materials put out by Joseph Garcia because they seemed to be the most ASL based (with some modifications). We also took a class (3 Saturday mornings, 2 hours) WHILE I was pregnant. We found out about the class by looking through the handouts/postings at Babies R Us. While I found great value in taking the class, I don't really think you need to buy a system. Just get a book to learn the basics of teaching sign, which is pick a sign or two you want to teach (I'd recommend "more" and "milk"), then say the word and do the sign every time you encounter that concept. Add more signs every week or two. There are two excellent ASL browsers that I use to look up signs:
http://aslbrowser.commtechlab.msu.edu/
and
http://www.aslpro.com/
Keep in mind that there isn't usually "one right way" to sign something and that ASL has dialects just like English does. Depending on where a deaf person was educated, the signs may be done differently. This website has some good information about deaf culture and using sign and also has a good dictionary: www.lifeprint.com
I also really like the Signing Times shows. I taped them off PBS, but I think they are available for sale (you might also check your local library). They've really been more for me, so I can learn the signs and then use them with my son interactively. He never really took to watching them, but it's been about a year, so I might put them on again and see if there is any more interest there. They will probably be helpful to you, though.
Signing has been really great for us and I know that it helped cut out a lot of the frustration of late-babyhood/early toddlerhood when my son couldn't say what he wanted, but he knew the sign. He just turned 3 and is starting to read and spell 3-5 letter words. It's funny because he gets a word a lot quicker when we finger spell it.
So, I say "go for it" and have fun. It's great that you are looking into things early and getting your ducks in a row. You're a great mama!