We've been using sign language with Nathaniel since he was four months old. Dave and I use sign language during our everyday conversations with Nate so that he can learn the signs and use them to communicate before he has the ability to speak. Using sign language with babies has a number of benefits including improving vocabulary and lessening the frustration that comes with not being understood.
As I've mentioned here, his first sign back to us was MORE at about nine and a half months old. You sign MORE by tapping your finger tips together. Here is Nate signing MORE at dinner when he wanted more cheese.
He's just starting to sign CHEESE as well, but I wasn't able to catch that on video. I tried to explain to him that he was my circus monkey and should perform, but the boy has a mind of his own.
Nate doesn't always sign MORE "correctly" - he makes several approximations for MORE including tapping one finger to the other palm, or one finger on one hand to several fingers on the other. In the beginning he would sometimes clap when he meant more as well. From what I hear these are pretty common variations. Just as you would do when speaking to a child, I acknowledge his attempts at communication whether the signs are "correct" or not and simply repeat the sign correctly.