Train your core with proper breathing!
This may come as a suprise to some of you, but the diaphram (your breathing muscle) is part of your core! The very top muscle of the core to be exact. And, order to have a well rounded, strong core, you should be training this muscle as well.
Here is how to do it!
1) Take a deep breath in. Focus on that diaphragm slowly lowering. Try to drop it down as far as you can!
2) Hold that breath for a second.
3) Slowly breathe out and visualize the diaphragm moving upward.
Protip: do not move your shoulders while breathing.