I started with all 3 of mine around 8 months. Some people say they get results early on, but most of my kiddos didn't start signing until 10-11 months, and that was just 1 or 2 signs. But right around their 1st birthday, all 3 of mine really started picking up a lot of signs. Then they began trying to speak the words with the signs, so I really think it helps jump-start their speech. It has really been most beneficial with my 3rd child, who is 22 months right now and hardly says a thing! But he can sign and use gestures to get us to understand most of what he wants to convey.
I didn't use any specific program. I bought a couple of second-hand books showing signs, and printed some information from the internet, even learned a few from friends. It's all about learning a few signs, doing them in context (sign "eat" right as you put food in their mouth, sign "down" every time you put them down). Repetion, repetition, repetition. Then after a few weeks, start saying & signing in context, then take their little hands & help them form the sign. Do this every time. Say it, sign it, help them sign it, praise them afterward. It might feel like you're getting nowhere, and then one day...you'r little baby signs for something & it's sooooo cool!!
Our most used signs (in order of most useful & eariliest learned) are:
Down (kids are always whining and jerking around to get down)
Milk
Eat
More
All Done
Drink
Please
Thank You
Help
We even made up some of our own signs. I pat my chest and say "I love you" to my kids, and they all learned that very early on. Nothing is sweeter than having your baby look at you and pat their chest in response to you saying I love you. Also, the sign for "all done" involves using both hands swiping back & forth in front of you (more or less)...so we use 1 hand swiping to say "No thank you." That is usually used in response to one of the older children trying to force a toy on the baby or drag him to a spot to play. Rather than scream, I'm teaching him to swipe his little hand in front of him to say "No thank you" (and having to teach the older ones to comply!!). So, my point is...you can even make up some of your own signs or gestures to improve communication.
We haven't use a lot of signs, just the ones that work for us. And it really has been great. I definitely recommend it. Just do it a lot and be patient for the results. Good luck!!
C.