I heard the authors of the article above in Boston a few years ago. The article is pretty academic, but has some great information. The part about teaching imitation is particularly informative (I've written the authors and asked them why they don't have their research more widely distributed, and they offered to send me a few pdfs of presentations they've made - I haven't seen them yet but will try to get permission to link to them when I do!)
Anyway, they talk alot about teaching skills for imitation. Banging the table, rubbing your nose, patting your head - lots of silly stuff. Helping out if the child doesn't do it to get them to do it so they get the idea that when you do something, they should try to do the same thing. Our kids apparently have a deficit in imitation skills, and until they learn to imitate and really get the idea of it, imitating speech and sounds are going to be difficult.
After I heard the session I spent a lot of time working with Violette on improving her imitation skills. I know after I learned about how important that skill was toward getting her to do lots of other things and really worked on it I saw a big difference in her ability to imitate sounds (or at least try to!)
We played/sang "can you do what I do? I do? I do? Can you do what I do? Tap Tap tap (where it says "Tap Tap Tap we put in whatever we want her to do.)
Now I can say "do X Violette" and she knows what it means to do it. She doesn't always do it right away, but eventually she tries. The good thing about teaching imitation is it is a skill that carries through so many other areas - writing, speaking, drawing, movements, etc. all can benefit from strong imitation skills!