I've done this before, but it's a little difficult to explain in words... basically, whenever I fabricate a puppet, I don't pattern the fabric before hand, I pin the fabric to the foam head, and cut the pattern from there. Whenever I attach the fabric to the mouthplates, I tend to tug on the fabric in the corners of the mouth a bit, and sort of tuck the top fabric into the bottom fabric, and that creates that sort of "dimple" look you're wanting.