I have been deeply investigating this pose for about 6 months. Really, ever since this photo was taken of me last fall. Can you see how my pelvis was "left behind"? I am not tucking it enough. I had no idea! Photos can be very helpful! I discovered two reasons for my "lack of tuck": my psoas and a tight gracilis (leading to knee pain.) I will briefly discuss both.
The psoas is arguably the most important skeletal muscle in the human body. I won't say more here, it is worthy of its very own post. Suffice it to say, that when it is tight/weak, you get what you see in the photo (the pelvis tips forward creating an exaggerated lower back curve.) Here are podcasts to help you with those psoas muscles!
Release and Lengthen the Psoas
The gracilis is an adductor (inner thigh muscle). Mine is particularly tight and when I do bent knee backbends (bridge, camel, bow), my knee hurts, just on the lower inside quadrant. I also have several students with the same pain. I began to massage the attachment for the muscle (you can fnd the attachment by linking to the gracilis link. It takes you to a great Wiki diagram) and softening it just a bit has made all the difference!
Here's to a deeper bridge pose!