I remember trying to get M. to imitate actions with ABA therapy. The idea was that I would do an action (clap, whatever) and if she copied me she would get a reward. She did it (providing the rewards were of the edible variety) but it didn't feel natural or fun. It also didn't generalise very well - I didn't notice a general increase in imitation skills outside of therapy.
Since we switched to II we've taken a different approach to imitation skills (as you'd expect). I think it's working better. As an example, a few evenings ago M. initiated a music session (she drags her daddy to the piano and makes him play nursery rhymes). She also has a PECS choice board to help her select which song she wants. As she danced to the music I copied her dance moves (they tend to be fairly big gross motor actions such as jumping, rocking on left and right leg in turn, shaking head dramatically from side to side to make her hair flick). She was really interested in me copying her, started taking notice, giving eye contact, and starting new moves with a look that was definitely inviting me to follow.
When we'd got a good connection going for a while I started introducing a little Makaton. M. has never been receptive to Makaton, more of a PECS girl. To my surprise however, she copied my sign for duck! It wasn't perfect, her hand was out to the side, but that didn't matter. I really think that me copying her movements first got her in a relaxed and receptive place where copying me in turn became possible.