Clam Sit-ups

This is one of my favorite exercises. Start from the "bound angle pose" bending forward.

Then, keeping your hands over your head, bend backward into the "reclining bound angle pose."

You'll find that the stretch in the groin is maximal at about 15 degrees of angle, just before putting your shoulders down. Then, return to the first position. More than likely, you'll find a sticking point (again at 15 degrees) where you have to use momentum to get past it, but try to do it slowly and smoothly. Repeat twice; you'll find you can stretch further forward each time.