Supermans are good for back strength too, with or without the ball. You go on all fours, like a dog (or cat in firm girl's case
)
Start by extending your right arm in front, then slowly extend your left leg straight out behind you, so your flying like superman. Keep that pose for 10 seconds, then switch arm and legs and work the opposite side. I usually do 20 reps of this (working each side 10 times) You can also do that on a stability ball.
Also to help your back, again on all fours, arch your back like a cat, hold it, then dip your back down and look up, this should be done slowly, then keep repeating, I do it 10 times. My trainer calls it the camel and giraffe. He laughs at me, coz when I do the neck extension bit I always yawn, and he calls me the sleeping giraffe.
I'd be inclined to do firmgirls cobra and then choose one of mine, and mix em about each time you do abs.
A fitball is a great investment, I use mine at the gym, and swear by them!!!