HOLD THE PEAK CONTRACTION
Lifting a weight is one thing, but holding it there takes the challenge to a new level. That's the concept behind peak-contraction training, which can be done for any body part, as you consciously squeeze the muscle at the end of the range of motion.
Compare a set of 20 crunches in which you merely raise yourself against one in which you hold the top position for a count. Still too easy? Try some hanging leg raises where you hold your legs straight for a second at the top. The latter is much harder.
Sure, you may end up doing far fewer reps at the end, but you'll make the muscle work harder, which is really what you want to be doing. Reaching a predetermined rep target shouldn't be your primary goal when it comes to ab training. Quality and difficulty of movements are, and always have been, more important!