When working with beginning communicators I start using basic sign language right away. (My own boys are no exception to this!) I will usually relate the sign language to the basic need of eating and start with the sign for more. As the student grasps that these motions I am making with my hands mean something, then they will begin to use the signs also.
To help students learn I have sign language in various areas of my room and also as my students PECS pictures. Some of the words I teach include more, help, thank you, please, and sorry. Also during circle time I sign the days of the week during the days of the week song, colors and letters. Not only do my beginning communicators learn a way to express themselves, but my other students love learning another way to communicate.
Do you use sign language in your classroom? How do you use it?