Zoë is almost 7 years old.... and for her birthday she wanted to teach you and babies around the world the ASL alphabet: http://www.youtube.com/watch?v=QKfhmqonbhU Write a little not to Zoë, she will really appreciate it!
"Can you come help sign 'more' as I feed your brother?" The oldest brother comes over to the table and automatically changes his voice and demeanor to match that of his baby brother and begins signing "more" while saying, "you want more, here's more." His mom watches the youngest clap his hands and raises her two hands in the air to say "yeah, good job signing more!"
My youngest just starting clapping at 9 months old. He was actually giggling at himself as he noticed his two hands coming together to make this magnificent sound. In my head, I realized, wow, that's one "sign" of signing (him creating movements with his hands to satisfy his own desires... this time it was purely a desire of entertainment^_^).
Look for those cues of your child gaining more and more motor skills.
Look for those sign approximations too! Often those first signs may not appear to be a sign.
Look for consistencyin their hand movements.
Help create environments that allows for consistency such as having a routine where you sign eat before sitting in the high chair, sign eat as you go to the refrigerator to get the food, and sign eat as you prepare the food.
And although clapping is a sign separate from the sign "more," within the context of eating and wanting more, big brother was able to help encourage using your hands to not just make fun noises but to put your hands together to communicate "more."