My friend started doing this when her bub was quite young (eg. "all done" at the end of a meal, "more", etc). By the time she was 18 months, she knew 75 words!!!! (either spoken or signed - not 75 signs). Anyway, it has definitely inspired me to do likewise with DS. We have started with "all done" after a meal, but I'm planning to introduce others when he's about 8 months.