Victoria,
I took a baby sign language class at North Lake college when my daugher was 5mo old. It was too early for her, but the class was really centered around teaching the mom to sign so it was good. My mom went with me, as she keeps my daughter during the day when I'm at work. We both worked with her on simple signs that we could be consistent with - Eat, bath and more. My daughter picked up on eat first. She recognized the sign meant getting her bottle and would make a rudimentary effort by 7mo to make it to us when she was hungry. Bath came next - probably because she loves the bath so much! - and she will ask to have a bath by coming to us and making the sign. I think she started that about 9mo. She just recently started the 'more' signal, about three weeks ago or so (just about a week after she turned 10mo... she just turned 11mo over the weekend).
My mom was actually more consistent with these than I was, so I credit her for my daughter's ability at this point. However, about the time she picked up 'more', it sorta clicked for her that she could 'talk' with her hands and now makes all kinds of gestures that we have no clue what they mean yet.
So, I signed her up for a sign language class at FastTrack Kids. This class is geared towards babies. We've only been to one so far, but it seems good.
One thing they told us at class was to limit the signs to about 6 new ones a time. Pick six signs to start with and when he picks up one, add another to your reportoire. They categorize the signs as "motivational" and "see-alot/do-alot" signs. Motivational signs are things the baby is motivated to learn - like play, eat, bath, etc. And the others are thing that you do alot with baby, that maybe you want them to learn that would be beneficial - like 'all done', 'change diaper', things like that. They suggest to pick three from each group to start.
I bought 'Baby Signs' by Linda Acredolo & Susan Goodwyn. I liked it because they explain the way the baby learns and have pictures of some of the children doing the signs. Oh, that' something else that they mentioned at the class... don't expect the sign to come back to you exactly as you make it... some movements are difficult for babies, but it should be close enough for you to know what they are trying to sign. They warned that it's easy to then try to sign back at them the way they making the sign, but just like if the baby says "bah" for "ball", that you continue saying "ball" correctly back to them, that you should continue to sign correctly for them. They are actually trying to make the sign the way you are and it can be confusing when you change things on them.
They gave us a book and some flash cards that are great and they also gave us a video, but I haven't looked at that yet. Maybe you could find these materials on craigslist or ebay. Also, we have two of the Baby Einstein videos that have signs on them.
Ok, so that was a very long post, but I hope it was helpful. Please feel free to write with any questions. I know that not everyone can take a class, but I would be happy to share the knowledge that we get from it with you.
Good Luck!
P.