When doing cardio-
Studies show that including interval training in your cardio sessions is proven to diminish stomach fat. If you find that you normally stick to a consistent pace when running, biking or swimming, mix it up by adding in sprint intervals into your sessions.
High intensity interval training (H.I.I.T) works on the same principle as intervals, where you push to your maximum training for a period of time and follow with a shorter rest period.
When strength training for abs-
Basic crunches can help tone your abs, however they only target certain muscles. Try incorporating full body exercises such as different plank variations (such as balancing on one leg or 1 arm). Also try using different pieces of equipment such as a weight or a stability ball.
Using both ways of training and eating a diet rich of healthy proteins, fats and carbohydrates will help decrease the amount of fat you carry around your torso.