Yes, we did sign language with Sabrina and plan to do it again with Cassandra. I have great memories of Sabrina signing her first words ("milk" and "more") and learning to communicate way before she was able to speak. We had lots of serious books on the subject, but the most useful turned out to be a series of board books designed for kids by language experts Linda Acredolo and Susan Goodwyn. The books cover basic signs and feature colorful photographs of babies signing each word, which appeals to young children. Another signing product we liked was this neat chart that helped us, our nanny and anyone visiting sign.
After Sabrina started speaking we dropped the signing and put away the books. I pulled them out again when preparing for Cassandra's birth, and to my surprise Sabrina was very interested in re-learning sign language. She is now able to sign words and phrases that she could not at 18 months and I think will be excited to sign with Cassandra when the time comes. Her favorite sign by far is "I love you," which she flashes me with throughout the day. Sometimes I even get her version of "I love you very much": two "I love you" signs with both hands in the air.