The best way to train the abs in my opinion is by forcing them to work. Reflexive training is nothing new, but in my opinion, not used enough. I have many patients that are constantly asking me on how to get their lower abs stronger.
One of the many dysfunctions I see is the psoas muscle substituting for the lower abs. This is really common in people that pursue Pilate's and gym goers that do lots of hanging leg raises.
Goblet Squat. Hold a dumbell or kettlebell close to your chest. This forces you to keep your chest up. Now as you squat down, drive the knees open. Weight is on your heels. At the bottom position, press the weight out, so your arms finish fully extended in front of you. You will feel the lower abs kick in big time. Bring the weight back in and drive through the heels, stand up and squeeze the glutes together. You just have done a tremendous exercise. You are learning to engage the glutes with the lower abs and providing mobility to the hips. In addition grooving a clean squat pattern that will have carryover to real life situations. How's that bang for your buck?