I can`t for the life of me figure out how to view your IM name...
Ah well.
How long have you had this cockatoo? How big is the cage now? What have you tried so far to gain her trust?
I would recommend sitting by her cage every day, not even necessarily looking after her, if that makes her uncomfortable. But sit by her cage and talk to her. If you run out of things to say, bring a book with you and sit there reading it outloud, in a sweet and childish voice. Play some music and try singing to her. All those sorts of things.
Try offering her her favorite treats from your hands, if she`ll allow that. If she won`t, I would say don`t put her favorite treats in her bowl with her meals... Just come by the cage later in the day, make a bit of a fuss over what a good little cockatoo she`s being, and tell her you have something absolutely WONDERFUL for her, show it off, and then drop it in her bowl. I think the best thing you could possibly give her to earn a little trust might very well be some warm baby food. (Warm, not HOT.) Try feeding it to her from a spoon. It might take a while to get her to trust you enough for that, though. If you have another bird that would eat from the spoon for you, I would do that in front of your cockatoo to model the behavior. She`d be more willing to give it a try. If you don`t have another bird to serve as the model, then you can spoon feed a human. Make sure your subject shows great pleasure and excitement to recieve such a treat! It`s just pureed veggies, so just buy a flavor your human subject would approve of.
You can also use any other birds you may have to serve as an example of good play-time behavior for your new cockatoo. Have an absolutely wonderful time cuddling and playing with your birds, and offering them treats, right in the view of your new cockatoo. Birds learn much faster when you have another bird to model the behaviors you want to encourage.
I`ve never had experience taming a cockatoo before (I usually work with the smaller birds), but I`ve had success with conures, cockatiels, lovebirds, budgies and senegals using the `towel technique.` Wrap the bird up gently but snuggly in a towel and stroke them gently. Keep the eyes covered to keep her calm, but try to uncover the top of her head so you can preen her crest and possibly rub her beak. Sometimes this is all a previously hand-fed bird needs to remember how much they like interacting with people. I know people who`ve had success with this, even with wild caught birds. They just need to remember how important physical interaction is to their happiness.
You have to judge, however, your cockatoo`s comfort level. If she growls at you when you meerly look at her, perhaps she wouldn`t calm down enough to enjoy the scritching. They need to have some level of trust and comfort with you already to calm down enough to enjoy it.
It can take months, or in some cases even years to earn a bird`s trust after they`ve been abused and mistreated. So please be patient and don`t lose hope. The rewards of taming, or re-taming a bird that no one else would have given a chance are tremendous. Consistent and persistent work is needed. You might want to consider consulting with an avian behaviorologist if it seems too much for you to handle alone, though that can get a little expensive.
I would also recommend reading books on bird behavior/taming/training. I`ve often found Sally Blanchard`s `The Companion Parrot Handbook` useful. Others feel free to chime in with behavior books that`ve helped you and your birds.
Good luck, and thank you for giving this poor thing a better life!