At one-year old and counting, my son can communicate so much more to me with signs than he can with verbal cues. Put the two—signs and sounds—together in context, and he and I can communicate smoothly almost all the time. Helps reduce frustration on both ends and heads of tantrums. Love it!
Here's a great kit for starting sign with your babies: SIGN with your BABY - Baby Sign Language (ASL) Kit: Includes Book, How-to DVD, Quick Reference Guide.
It includes an instructional DVD for parents, a book with sign dictionary, and a laminated quick-reference sign sheet (which is handy to share with sitters).
I also highly, highly recommend the Signing Time DVD series for toddlers and preschoolers. My kids love it! (My husband and I don't mind them either, although we think Rachel, the hostess/teacher, becomes more confidant and polished as the series goes on.)
There are two seasons with multiple volumes per each. There is a third series, Baby Signing Time, but I don't like showing babies a lot of DVDs and I like the toddler/preschool ones better. Here's season 1, volume 1 as well as a couple package options to get you started.