I agree the key is totally to intervene and bring the child back down before they get to that tantrum place. Once they've reached that threshold, I have found that all reason is useless. But to catch them before they spiral off requires so much watching, and predicting, it just can't be done all the time.
I feel like my DS is making great progress in being able to stop himself from following through with an aggressive impulse, or from having a meltdown, he also calms faster once he has reached a tantrum. I credit it all to O.T. His O.T. makes a strong emphasis, and I don't know if this is unique or not, on having him engage in a cognitively stimulating activity while he is doing sensory activities. She explained that it helps his cognitive take control over his more primitive (reflexive, emotional, fight or flight) brain function. I think that is exactly what I see improving.
Another thing that we just started, and today is the first time I actually see DS using this technique is to try to change his thinking about things that happen. For him, many things are a big deal. The truth is that very few things are a big deal so he need to learn to take the things that bother him, make it a small deal, and let it go. It helps to repeat it a lot "let it go". We also use visuals, like "it's small like a bug, and he plays with a toy bug, and it is easy to let it go". And we use hand motions to show letting go. He didn't start using it until today, we decided together that a chart might help. He gets a point for every time he lets something go, then after so many points he'll get a game that he really wants. I'm hopeful because I see that he is excited at the fact that he CAN do it. I never appreciated before how much he wants to have control over his aggressive impulses and for him it is just too hard. It must feel pretty bad to feel that out of control so often. I picked up this approach from a book I am reading that has given me lots of new ideas for changing his behavior. It is Parenting Your Asperger Child. He does not have an official Dx of diagnosis, but the techniques make more sense for him than anything else we have tried.
As far as helping her calm down when you see her ramping up, I think it is best to remain as calm and loving as you possibly can. Sometimes I don't feel at all like giving a great big hug, but sometimes that does the trick, and as soon as I feel him mold into my arms, I never want to let him go.