I am a sign language interpreter. I have my first little girl who is 8 months old. I am currently using sign language with her. She seems to understand what I am saying but she does not sign back yet.
Though I just know the language, and am currently learning how to use it with babies...I feel like the most important thing is to be consistent. Find a handful of signs and use them every day, every time. i.e. Food, Milk, Water, All Done, More, Mom, Dad. These are the ones I am using right now. I think she understands all of them except "all done." She isn't yet signing but I have no doubt it will come as she develops her motor skills. Right now just the fact that she understands me is so exciting.
Also I have heard GREAT things about "Signing Time" DVD's. I have yet to use them myself because I don't think my daughter is quite to the point of watching TV yet. I would recommend looking for them at your local library first that way you can try them out and see if they are something you want to use. I have checked them out from the library for my older nieces and they LOVE THEM!
Hope this helps!