Sit-ups can often turn into a momentum-assisted exercise, which means your abs aren't working as much after the first few reps. This static contraction forces your abdominals to work overtime—without any help.
To do it: Start sitting on a sturdy chair or stool with your knees pressed together and hands outside of your hips. Brace your abs in tight and press down with your arms to lift your hips and hover above the seat of the chair. Keep your shoulders down and focus on lifting your knees with your abs, not your legs. Hold for up to 30 seconds. Work up to 3 sets of 30-second holds.