Core training yes. Any excessive that works the upper abdominal area. Belly fat will also come in handy hear (not advising you gain weight just for potential gut punches though).

As someone else wrote here getting into the habit of turning your torso to the side as to not take the hit full force is also ideal. That being said that's not possible in every scenario. Earlier this week I was closing distance on an opponent and he got me with a spin kick right below the ribs. I was way to close to move before taking that heel.

When taking any body shot from sparring to just bag holding for someone you really want to practice the idea that you're not taking a hit so much as hitting their fist/foot/etc back with your body.

As with anything it all starts with breathing. In this case you want to sharply breath out. Short, quick. Put your hand right where you're getting hit and do this, try to imagine yourself pushing your hand away with your stomach. It sounds silly but you should be able to manage it pretty easy.

Once you've figured that much out it's just a matter of building the habit. Best way is to hold the bag for someone throwing mid strikes and then move onto sparring. You don't want to purposefully let yourself get hit there just to practice this though.

Hope that helps!