C.S.
I don't know if I'm any help or not, but we struggled with this with my son, also. If there's a certain time of day that she's in the best mood, it's easiest to start there. One thing we did that made a huge difference is that we started with somewhere that has a more open studio (rather than a smaller back room- it can be very scary). We also started with a cheap-o studio, one that has the $3.99 sheets or the $6 package or whatever, so it feels like less of a loss if it doesn't work. If it does, great! You got a few decent pictures out of it. But, when you start with somewhere like that, it's also easier to just give up and let them give up. You can do it as a more casual picture, too. For us, we let our son dress up in his favorite Buzz Lightyear costume for it, just to make it "fun." Once he saw it was all okay and not that bad, and he got to be done when he was done, it made a TON of difference when we went to get the "real" pictures taken. Also, find somewhere that you like the photographer. If the photographer is impatient or in a hurry, that's going to make it worse.
One more word of advice- I used to do children's photography- if you get stressed out and worked up, it's going to make it that much harder and it's all over. Let them bring a favorite toy or book or whatever, and make it seem like it's the most fun thing they could possibly be doing. I know that's much easier said than done, especially when you've spent time getting them all dressed up, but it really does seem to work that way.
Don't know if that was too much, but it's what I found to be effective. Good luck! I know it's tough!