Read Max Strom's response:
You're not alone; many people find this move difficult. Here's a technique that might help. As you move from Downward Dog, rise onto the balls of your feet, drawing your heels up and raising your buttocks as high as possible; then keep the right foot in this position as you step forward with your left leg. As you lunge, exhale, pull your belly in, try to touch your left knee to your chest, and then place your foot as far forward as you can. Elevating your hands on blocks can also help.
For more progress, you need to build strength in your hip flexor and abdominal muscles and flexibility in your hamstrings and hips. So include hamstring and hip stretches and core strengthening exercises like Paripurna Navasana (Full Boat Pose) in your practice.
Traditionally, Paripurna Navasana is done with the spine and legs straight and the arms reaching forward from the shoulders, so you look like an upside-down "A." But if your core is weak, that version may strain your lower back. Instead, practice with your hands behind your knees and your knees bent. As you get stronger, you can straighten your knees and let go of your legs. But if either of those additions rounds your lower back or makes you lean back further, stick with the more basic position.
The co-founder and former director of Sacred Movement Yoga in Los Angeles, Max Strom is the creator of the DVD Max Strom Yoga: Strength, Grace, and Healing. Visit www.maxstrom.com.