Cut a short foam roller in half so you have a half circle. Lay over it so it's in the natural lumbar curve of your low back. Try to expand your stomach and rib cage laterally as you lay back over the roller. When you sit up, push your stomach out hard like you are squatting. No matter your stance on bracing, pushing out etc during squatting; do this it will make the movement harder. Also, try to roll your hips toward your face on the way up. If you have a room full of 20 athletes who can't grasp these concepts, just have them crunch over the roller, it will still fry their abs!
Do at least 100 total reps. Hold a weight under your chin to make these harder.
Train your abs every day your train! These may look simple and basic but they are very effective.