Mushy mid-sections have been the target of many a crunch, sit-up, side bend, etc. for generations–but I have to tell ya those types of exercises are missing the point when it comes to strengthening your core for its intended purpose.
Your core is essentially a sphere made up of different muscles that lives between the base of your rib cage and the basin of your pelvis. While the aforementioned exercises do address one function–initiating movement in a variety of directions–I would argue that these muscles are better suited to RESIST aberrant movement of our low back that causes movement inefficiencies and exposes us to injury and pain.
If you intend to strengthen your core to improve its ability to do the things I just mentioned then the types of exercises you choose to do so become very important. Exercises like planks (including side plank!), dying bug, bird dog, and the pallof press are all good examples of movements that do a better job of training this crucial aspect of core strength.
The other problem with traditional core exercises is that they can also expose your spine to repetitive use strain and injury. These types of exercises tend to be dosed out in high numbers and if you can imagine the ligaments and tendons in your spine acting somewhat like a paper clip does when you bend it back and forth, then you likely get the point I am trying to make.
For the more functional type exercises the key is to getting your spine, pelvis, and rib cage into an optimal, neutral position, and challenge yourself to maintain it while doing them. If you would like more of a demonstration of that concept, click here to watch a short video. The other important skill will be learning how to brace your core which you can learn more about by clicking here.
Ultimately all movements should be initiated with these ideals in mind, and with that you should notice improvements in your posture, strength, and efficiency.
If you have any other questions, please feel free to reach out anytime!