Pelvic tilts.. I think that's what they're called. Or bridges. Whatever.
Get into crunch position, laying on back with knees bent. Lift your pelvis upwards and hold for a count of 30 seconds. To make this extra challenging, try using a balance board ( I bought a golds gym one at walmart for 20 or less) underneath your feet. GREAT for lower back. (one of my weakest spots)
Also, planks are great for strengthening the core and back. Or, if you have an exercise ball, a ball roll out. Kneel in front of your exercise ball and lean forward, resting your forearms on the ball. In a controlled motion, slowly lean forwards, moving the ball with you. You'll find yourself leaning forwards with your feet off the ground, also kind of leaning with you, that's all good. Keeping your movements slow and controlled, lean and roll the ball backwards.
Fighting back from the regain
Strong is the new skinny.