If the bulk of your core strengthening work consists of sit ups, crunches and other core flexing moves, consider doing more core stability or anti-rotation exercises like planks or this move called a Pallof Press. The trick is to pick a resistance that challenges you while still allowing you to move with symmetry and good posture (hips level, shoulders down, pelvis tucked). Start light with 2-3 sets of 12 reps each side and build. You will feel this all over especially in your core and glutes. These are particularly good if you have lower back issues and likely a safer choice than sit ups.