ROUND YOUR BACK
You probably won't see those words anywhere else in a training article anytime soon. That's because you've rightly spent so much time learning how to keep your back flat and locked in this position when doing dead-lifts, bent-over rows, squats, and other standing and bent-over exercises. The problem is that you need to unlearn that habit when training abs. Maintaining a flat or slightly arched back in the lumbar region in dynamic movements ensures the antagonist muscle the abs won't be able to fully contract in an active manner.
In fact, it's common to see individuals doing exercises like cable crunches and decline crunches with a flat back in which they're bending at the hips, not the waist. When you bend at the waist, you're able to fully crunch the rectus abdominis, meaning it shortens; at the same time, the lower back musculature is being stretched. Maintaining a flat back and bending at the hips actually hinders your abdominal training because you can't fully contract your lower back and abs at the same time. All you get is an isometric contraction which has value, but isn't what you're doing a crunch for?
Sure, a lot of pundits will tell you you're putting your lower back at risk by doing crunches at all, but they're overstating their case. That lingering lower-back soreness doesn't come from simple crunches, but more likely the same ab routine any ab routine over and over again for months and years at a time.
The lesson for you: Diversify! Your back is made to bend, so do your crunches if you want, but be sure to get out of the sagittal plane for some rotational and lateral ab movements as well.