[ericbrow]

I'm going to answer this assuming you mean gaining number sense, which is something I didn't realize I was missing until I gained it.

I got my undergrad in math and physics. I was good at math. It wasn't until I had been teaching high school for 3-4 years when some gave me a copy of Shortcut Math by Gerard Kelly. After reading it and practicing the techniques, arithmetic made so much sense. I was able to easily add, subtract, and multiply larger numbers in my head.

Interestingly enough, many of the techniques taught in this book are also part of the common core math curriculum. It's a way to help students gain number sense.