Why it's on the list: This old-school fave amps up the ab engagement by increasing the range of motion over standard crunches. You can also dial up—or down—the degree of difficulty by adjusting the angle of the bench.
Holding a medicine ball, dumbbell, or plate against your chest adds a further level of customizable resistance. If you want the ab-chiseling upside of cables or gym machines but don't have access to a gym, this is for you.
Be warned: Since your feet are hooked, it's all too easy to use your hip flexors to come up instead of your abs. Keep your lower back flat to the bench and your abs engaged. If you're not feeling a wicked burn, drop all the weight, put your hands on your belly, and really focus on the contraction at a slower pace.
Decline Crunch Variations for Ab Development:
In your workout: Position this one later in your routine after you've built up some fatigue from a really challenging first exercise or two. Do 3 sets of 10-15 reps. If you find 15 reps too easy, simply increase the angle of decline or use a heavier ball or plate.