Robin and I bought Joseph Garcia's ASL book Sign with your baby before Lorenzo was born and have been working on Milk and Toilet off and on with him. We also do Cat to refer to our cat Scooter. Garcia suggests starting to sign around 7 months, but we've been doing it anyway, mostly to learn the signs ourselves. A while back I also came across a great site called HandSpeak with animated gifs showing how to sign. There is a nice section with signs specifically for baby.