Look up Tony Gentilcore's articles on dead bugs, plank variations, and pallof presses. Very knowledgeable dude and keeps his articles short and to the point. The other big one i have recently started to incorporate has been front squat holds for time. Keep your glutes contracted and take a big brace then just lift off and hold the weight in the front rack position. I usually do 3x15 seconds and add 5-10lbs when i get all 3 sets comfortably.