Familiarity with curriculum content really helps flipping I believe. If you know exactly what the students are meant to understand, it is easier to provide them with the exact direct instruction in the video and also a suitable activity in the classroom.
What I found interesting was my new class were not so easy to sell on the idea of watching videos at home and using the blog/Drive etc to share ideas and comments. My last class had been much more eager to watch and comment on the videos. I had to really push my new lot to watch the video. However, the actual activity was great with them all collaborating to make fruit cocktails in different proportions and reminded me of how powerful a tool flipped teaching can be.
The children loved making their own cocktails, tasting them, naming them and then drawing up advertising posters which showed the proportion of different juices in them. They also had to work out how much of each juice they would need to make more or less of the drink. Great fun.