I recently stumbled across a brilliant math teacher leader who you will want to hook up with. Her name is Christina Tondevold from Build Math Minds. You can find her on Facebook, twitter, and her website buildmathminds.com. I really can't say enough great things about her.
She's sharing a lot about number sense right now and is offering a four-part free video series on the topic. Two of the videos are available now, which I've watched and LOVE. The other two will be available soon. The catch is that they won't be available forever. If you're going to watch them, and you should, you'll want to start watching soon.
So go here and get busy.