Hi S.,
I taught my daughter sign language (and I plan to do with any other kids we have). We don't have hundreds of signs...but we have about 20 or 25 basic signs for things that we use/do every day. I can't tell you how much it has helped reduce whinning and frustration.
I started when my daughter was about 5months old. At that time their vision is getting better so they can see better. I started with milk. I nursed my daughter, so every time I was getting ready to nurse her, I would sign 'milk'. I would also do it once or twice WHILE nursing her. I did this each and every day every time I nursed her.
My daughter didn't start signing back until she was about 9 months old...but she DID understand me much earlier then that. I would say definetely by 7 months she KNEW when I signed milk what it meant.
At about 8 months I threw in 'more'. Once she started signing back, I added about 1 new sign per week.
My daughter is now 17 months and I do find times to add more signs. Now, I will show her once, maybe twice and she's got it.
Keep in mind, OFTEN, babies will modify the sign because their hand/finger dexterity is not like an adults. For example, the ASL sign for water is to make the 'W' sign and then tap that on the SIDE of your chin 3 times. My daughter open her whole hand, fingers wide and taps her hand over her mouth for water. So, they will vary it ... but you know what they are trying to say and quite honestly, I find it really amazing what they come up with!
"Sign with Your Baby" is a great book to get started. Sorry I don't have the author right here, my sister is borrowing the book. Look it up on Amazon, you'll find it.
Also, www.lifeprint.com is a good site. They use ASL signs.
Some moms make up their own, but I figure why not just teach the ASL signs. Then they actually KNOW another language.
I've had some people ask me if it has delayed her speaking in any way. I don't feel it has. My daughter is very chatty and speaks many words. She isn't talking AS much as some other kids her age...but at the same time we are also a 2 spoken language house. My husband speaks Dutch and I speak English. So, I have read and our pediatrician has told us, that can delay things. They are just processing SOOO much.
The other thing I have noticed is there are a great many moms these days who are using sign language to some extent. So, often times when we go to play groups and such the OTHER moms will know what my daughter is asking for even if I don't see her signing something! It's such a great help!
My advice, once baby starts signing, be ready with new signs for things you use/do often. Frustration WILL set in with your child if they can't speak something and can't sign something and need to tell you something. I saw that with my daughter and it inspired me to learn more signs. She knew she had a means to communicate with me...but couldn't speak the word and didn't know the right sign...so she would just start signing all the things she DID know...but it was never what she wanted.
At first I thought she was just being obstinate...then I realized she was TRYING to tell me what she needed...but didn't know how. So, be ready. Kids are amazing and when you teach them something they will RUN with it!
Good luck! And congratulations on being a mommy! The adventure is wonderful.