All the research says that baby sign language encourages effective communication; helps reduce frustrations, and is eventually replaced with words. I see it every day. I always use and encourage sign language in my speech sessions and I am becoming quite the signer. I have learned lots of new signs and this is my personal sign bank.
If you're a therapist, I encourage you to learn relevant signs and use them in your sessions as you feel necessary. If you're a parent, I encourage you to learn and use some signs with your child if they are having communication difficulties.

Signing gives children an outlet if they don't yet have one. A child should first be able to point, wave hi/bye, raising hands up to be picked up, etc (early routine gestures) before they are ready to use signs. Use hand over hand and show your child what he is supposed to do. Always pair a word with the sign. And remember, toddlers are sloppy signers! They may not do it perfectly, but if they are attempting to, give them credit and praise!
If you would like to learn a sign listed below, print off flash cards, or learn more about signing, please visit one of the following: /www.babysignlanguage.com/ or /www.weehands.com/. If you know of a relevant sign for young children that isn't listed, please list it in a comment and I'll learn it and add it to the list! I plan on continuously adding to my sign bank and would appreciate new ideas.
I always start with: more, all done, milk, fish, help
Manners are always nice: please, thank you
Names for people: mama, daddy, me, baby, grandma, grandpa
Food: cracker, cookie, banana, yogurt, ice cream, water, apple, candy
Animals: snake, dog, duck, cat, horse, cow, pig, frog, bird, bear, monkey, sheep
Vehicles: train, car, airplane, boat
Nouns: ball, bubbles, book, bath, music, phone, blocks, diaper, shoes, star
Action words: want, open, work, play, eat, drink, night night, wash, sit, walk, potty, cut
Directional words: up, in, on
Colors: blue, red, green, yellow, orange, purple
Others: quiet, sorry, gentle, yes, no, hot