I use the ab machines at the gym. I do a couple variations on crunches (sitting up and lying down both on machines with weights. I can crunch 55lbs on the one that’s sitting position, 40 on the other), hanging leg lifts (which kill my shoulders), the oblique twist machine, as well as full body workouts. I just started the gym about 2 months ago and have really upped the weight since then so my body is definitely progressing.
But you can still go wrong. Because your feet are hooked, it's all too easy to pull through your thighs. Nor do you want to go all the way down to rest on the bench between reps; stay well off it. 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. You can also introduce a cross-body movement, angling your elbow to the opposite-side thigh, to better engage the obliques.
“For me, getting a six-pack was about paying attention to what I was eating and lifting heavy weights at CrossFit. I do have a semi-strict diet, and I loosely count macros (focusing on daily carbs, protein, and fat intake rather than blindly counting calories) to stay balanced and on track. It sounds like a lot of work, but it really isn’t. Eventually, it just became a part of my lifestyle, and it’s something I enjoy doing. I even started a custom meal-plan business based on macros because I had such great results!” —Tina Haupert, @carrotsncake