Back exercises we do in my class:
Stand in a squat position. Raise up your arms straight, palms facing each other. Now flap your arms back as far as you can and back to center. Switch your palms facing out and do the same. You should feel this in your lower back.
Stand in a squat position and hold a very large ball or other object straight up over your head. Hold it as high as you can for as long as you can. This will HUUUURRRRRTTT!!!
One day we did situps over a large ball. Sit at the end of the ball, lean back over the ball, do situps one hand to each ankle. Try to keep the chest high. Really made my abs sore like nothing else and did not hurt my back, so it was obvious I wasn't using back muscles to make up for my flabby abs.
Female, 5'3", 48, Starting weight: 163lbs. Current weight: 135.
Starting squat: 45lbs. Current squat: 145 x 4.