Make this exercise a staple of your lower-body/posterior chain training day:

Horizontal Hyperextensions with Stretch Emphasis

1. You want to tackle these with already pumped glutes and hamstrings. So, these would make for a great final exercise of your workout.
2. Instead of locking out your knees, maintain a small bend. This will allow you to initiate each rep as you would a lying leg curl for maximum hamstring stimulation, and a wicked contraction.
3. Use your hands to literally pull yourself into a deeper stretch. Breathe deeply and hold the stretch for 10-30 seconds. Then let go while flexing your hamstrings and squeezing your glutes like mad to complete the rep.
4. Depending on your hamstring strength and endurance, 6-10 reps should suffice. You can load these by holding a plate across your chest, a bar across your back, or by using bands.

If you struggle to connect with your glutes and hamstrings, or you lack mobility, integrating this movement into your lower body days will improve both situations in short order.