Engage your core during every exercise. Most exercises involve your core in some capacity — and even more so if you remember to squeeze it. And you burn more calories when you work larger muscle groups (your abs and back) than smaller muscles (like biceps), Cohen says.
To max out, engage all these groups at once — and try some moves that involve rotation, such as plank twists. (They're the human version of wringing out a towel — just imagine squeezing out the fat for a narrower, tauter waistline.)