The first sign to catch on was "more." We signed it as we fed them, and Alex, of course, was the first to mimic us.
You could see it on Alex's face: "Hey! I make that sign and they give me more food. Bring it, mama!" Alex will even use the "more" sign for things other than food now.
Isaac signs "more" once in a while, but mostly claps for more food. And Sam has stubbornly resisted doing the sign, even though he clearly knows what it means and is capable. And since we have to feed him even if he doesn't sign, (I know, Parents of the Year, right here!) he isn't very motivated.
The first time Sam ever pulled out the "more" sign was at the boys' first birthday party. He finished his first cupcake and immediately and adamantly started signing for more. Of course, we gave him another cupcake. It was just too funny!
Well the other day Amelia and I made cupcakes and yesterday I gave the boys each a quarter of a cupcake. (Not to worry, I stepped up the plate and ate that odd quarter.) Sam finished his and started signing "more!" He hasn't signed it since their birthday party in September.
Apparently all it takes for Sam is the right motivation!