Signing with the babies has been one of my favorite activities over the past year. H and I used certain signs for months before the babies ever signed back to us. They knew what we were signing for months, but didn't start signing themselves until they were over a year old. Right now they have a few signs they make, usually with prompting, though it is fun when they walk into the kitchen signing 
milk on their own accord.
My two favorite resources for signing are 
Signing Time and www.lifeprint.com. 
Signing Time is a television show for kids. We have the first book and DVD. 
This book is one of the babes' favorites. They know what some of the pages are asking them to sign. I like the DVD because it has two songs that we can sing and sign. The lifeprint site has the best explanations for how to make signs. The site was created by an ASL professor. Most of the words I look up on it are there.
What I love about this stage in our signing is to see how the babes deliver the signs. For example, here is how ASL says to sign 
please:
The sign for "please" is made   by placing your flat right hand over the center of your chest.  Move your   hand in a clockwise motion (from the observer's point of view, use a circular   motion towards your left, down, right, and back up) a few times. 
   
Lucia's version of 
please is to do the "roll the dough, roll the dough" of Patty Cake: rolling her two fists around in a circle. Every time we say "say please" she rolls the dough.
The sign for 
milk is to squeeze your fist as if you are milking a cow, kind of like a beating heart. Mazie likes to move her fingers individually, as if she were impatiently tapping her fingers on a table, instead doing it on her hand.
Sometimes I try to think about what the children are seeing. What do they think of all these dances we do with our hands? Two of the signs we use are very similar: 
more and 
shoes.
More:
Sign:  MORE  Handshape:  Both hands use a semi-flattened "o"  Location: Normal signing space in front of the body  Orientation: palms facing each other  Movement:  Inward. Bring both "flattened-O" hands together.

Shoes:
The sign for "shoes" is made by closing both hands, thumbs facing. Gently strike your hands  together twice.
       

The babies usually sign 
more for 
shoes. We've been signing 
more a lot longer (parents generally start teaching signs that pertain to eating) so that is the habit or what is in the babies' heads. I wonder what they think since the signs are so similar. Perhaps they think their parents are crazy. The babes will sign 
shoes; it just looks like 
more than 
shoes. And Lucia, who loves to use her pointer fingers for everything, signs 
more with her pointers, rather than all four fingers.
Elliott has his own version of 
all done. Here is how ASL teaches to sign 
all done:
       |   FINISH / over / done
  The sign for "finish" is made by placing both of your open hands in front of you.  Each hand should face you, with your fingers pointing upward.  Twist both hands quickly a couple times ending with the palms pointing  (somewhat) forward.  You can also do this sign with just a single twist  which makes it seem more "final."
 
  | 
Elliott's version of 
all done is to take both hands, hold them up in the air, and sign 
milk with both of them. He uses the 
milk sign a lot. I just told him to go pick up his book, and I signed 
book. Elliott, while walking over to his book, signed 
milk with one of his hands. He understood book, but I would love to know what he was thinking as he went to pick it up while signing 
milk.Have a good day and 
     
.