Learning sign language is just like learning other languages, and the earlier you start teaching your children, the easier time they'll have with it. When I was a kid, my mom was learning Spanish, so she decided to teach us as well. Flash cards probably didn't work as well as she hoped, but incorporating often-used phrases helped us learn some basics. Incorporating signs for certain words whenever you say them will help your baby pick up on the meaning, and eventually when they hear and see it enough, they'll start using the signs themselves. Here is a collection of board books, DVDs, and books for adults on baby sign language. If you're looking for other resources, OverDrive has some eBooks and a video available. If you want to learn a language other than sign language, Transparent Language is a great eSource (that has lessons for children called KidsSpeak for certain languages).
Books for Adults