In case you missed my first post on perfecting your plank you can check it out here. There I talked about the importance of setting up your spine.
The thing is, it's hard! Your spine is like a flexible rod. And during a plank, we want to momentarily suspend the "flexible" part, and find a strong, flat spine from the shoulders to the hips. We have to also momentarily make our low back curve disappear by rotating the pelvis, tucking the tailbone toward the nose, and engaging the deep low abdominals.
Sound so simple! But it's not easy to make these micro adjustments.
Enter the foam roller. By placing the roller on your back (you might need help) you'll have feedback. You'll be able to feel where your spine is in space.
First image: spine is neutral and you can see a pocket of air between the roller and my low back.
Second image: you can no longer see any space between the roller and my spine.
Once you've found a flat back, try to lift your knees and extend your legs, still feeling every bone of your spine from your shoulders to your hips keep in contact with the roller.
Give it a shot! Hopefully it helps you to find your form and further perfect your plank.