just to add to pierre's epic post--work on some hip flexor activation drills to help actively pull yourself to the bottom--it will both improve your depth and your lordotic maintenance.
for example, lie on your back with the lumbar arch intact. bend your knees to 90 degrees, raise the thighs to vertical. have a partner hold your feet and attempty to pull your legs (straighten them) while you attempt to pull your knees to your chest. your partner should provide plenty of resistance, but should slowly allow you to get your knees to your chest. make sure not to let go of that lumbar arch in the process. throw in a couple of those immediatey before a set of squats and see what happens.
that drill from pavel t, by the way.