A mom over at BabyCenter.com has asked some great questions about signing with her baby. I remember visit BabyCenter.com years ago when Joshua was first born...he turned 11 years old yesterday! Here is my response to autumnb071's questions:
Signing with young children is helpful because the motor skills needed for signs develop before the motor skills needed for speech develop. First spoken words develop between 12 and 14 months, typically, while first signs can be produced by baby between 9 and 12 months of age.
Share Visual Information
As well, your baby will understand what you sign to them before they can produce their own first sign, e.g., they'll calm down when they see you sign MILK or get excited when you sign BATH. The visual information you'll be giving them, helps them understand your message. This is similar to our use of gestures when we give directions...I understand where a restaurant bathroom is better when a server points to where it is in addition to telling me using words.
The motivation behind any behaviour...for anyone is either (1) to make requests, (2) to get attention, (3) to escape a situation or (4) for sensory reasons. Three out of four reasons for behaviour is communication, and yes, that means 3 out of the 4 reasons for tantrums are communication based, e.g., I want food, I want you, I don't like this anymore.
Being able to communicate these needs clearly before speech develops, or when you are so emotional speech doesn't come, will lessen challenging behaviours.
Easy to Learn
Babies learn to wave because you model that gesture for them, encourage them to repeat it and reinforce it. So do the same with signs for things that they like, e.g., milk, food, water, etc. When I was learning Italian (and I still am) I learned the Italian words for things that I liked and were important to me the fastest! (Une bicchiere de vino rosso, anyone!)
Use signs for things that they like and that they can feel/taste/touch/smell to start with. Highlight their sensory experiences with signs and speech ... and narrate their world!