If you handle a bird, there is no way to prevent a bite. I agree, no shoulder time for a bird who bites. But the problem is, you can have her on your shoulder 30-40 times without and issue and one day, the planets align, the moon is full, you wear the wrong colored shirt or she see's you interact with someone else and desides to bite. They are animals and there's really no ryme or reason. They could just be in a bad mood.
But with your bird, I'm thinking there may not be a compatability between the two of you. Some times, no matter how hard we try, we can't make people like us and it's the same thing with animal. I know my Eclectus is that way. I can have people over to the house visiting and Mac will go to all of them except a couple of folks. If I place Mac on their arm, he will jump off or start flapping his wings until I remove him. Thefolks feel bad about it and I alway make excuses like well, he doesn't care for the color of your shirt, or he doesn't care for red headed people with beards hahahahahah. But I know Mac just doesn't care for those few people and there's no way I can change that.
I'm not one of these people who will tell you "DON'T REHOME HER". I'm a realist and believe that sometimes it's for the best for both of you. I can tell you right now, If I have a bird that is constantly biting me, I will rehome them quick, fast and in a hurry because that shows me that you either don't like me or don't want to live here.
Mac bit my left ear pretty severely a couple of weeks ago and fortunately for him there were no stitches or hospital bills involved because if there had been, he would most likely be living someplace else right now. I knwo why he did it, I took him off of my wife's shoulder and he was trying to get frisky with her and she wanted him removed. But it still doesn't excuse what he did. His only saving grace is that I didn't have to go to the hospital. But his beak did puncture my ear above the earlobe piercing it through. It's still a tad sore to this day.