There are a few exercises that work great for your back that you can find here. Hip-ups (aka bridge), super mans (aka locust pose), single floor leg lifts. I used them in my workouts (Core Blast, Sexy Booty). Also checkout ab-workouts-guide.com/core-exercises. You there you'll find corkscrews, the wheel, and reverse snow angels. They're pretty effective, I use em in my routine.
Good luck and God Bless!
Supermans with 2 or 3 lbs in hands and/or ankles. Depending on your strength try 5lbs. :)