Our stake had a Best of Enrichment Activity on Saturday, and one of the classes I peeked into was about baby sign language. The teacher listed a bunch of websites on the board, and I thought I'd pass them along. We've done a little signing in our home, but I haven't been really dedicated. The sign I see with some consistency is "all done" and occasionally "more." Those are both helpful at meal time!
Apart from Internet sources, what I've found most helpful have been the books (for adults and babies) and videos available at our local library.
Smart Baby Sign Language
Sign With Me