Hi J.,
I echo the sentiments of all the other mamas, but wanted to also offer this practical advice for changing behavior. You're going to freak her out. I think it's all fine and good to go with the you are the mom just say no perspective, but in addition to the advice you got about thinking about what kind of person you want her to be, this is a great teaching moment to teach HER how to change HER behavior WITH you as you change YOUR behavior.
First - hubby's got to be on board or this can turn into a nightmare. I'll assume he is.
I would NOT start out with what I like to call the smackdown. The smackdown is for when someone gets out of control over the top and YOU haven't contributed. In this case, what's your goal? To change the behavior. Is your goal to create chaos and resentment? NO. Your goal? Change the behavior. You're dealing with an 8 year old not a 2 year old, so I would treat her like an 8 year old.
I would start out with the (validate/condition/explanation) style. I just made that up - it's not a real parenting philosophy. just what works with my kid.
So you're at the mall and she wants a shirt.
"Mom I want this shirt"
Validate
Yep - that is a cool shirt. it would look great on you.
"Can I have it?"
Condition
"This trip is not about purchasing clothes, so we will save clothes shopping for xxxxx".
"But I want this shirt"
Explain
"I know. It's a cool shirt. I think next time we go clothes shopping we will set a budget and then we can pick stuff out. If you like that shirt more than the others we see in the next (whenever it will be until you are planning to buy her more clothes... summer? so a month?) we can get that one"
Notice you said no. But not really. I mean you're going to buy her a shirt again at some point. She grew from last year. So, it's reasonable that she have some influence over what she picks out and can start learning how to make those choices for herself. You didn't just say no. You let her know under what circumstances she could have what she wants (not what she needs - you still provide that).
here's where it will get dicey. She'll either buy in or get mad. Probably get mad. If she starts to misbehave, just deal with her emotion without your emotion.
"I understand that you are upset......
what do you hope to accomplish by speaking in that tone?"
Is this really important enough for you to cry in public?"
Why do you think this shirt is important enough for you to have right this second?"
Then you have to decide how you handle it when she throws an 8 year old tantrum.
I also think while you are doing this now is a good time to assess what she CAN have control over. If you start giving her some leeway she will take a loss of control in other areas better if there is a clear diving line of what she does control. I don't buy my daughter anything that is not appropriate so she has FREE REIGN to wear whatever her little 10 year old heart desires and I keep my big fat mouth SHUT. We do have "Mom pick events" and she knows that - certain things where I dictate what she wears. And she's ok with that, cuz it's like 5 times a year. The rest of the days it's all her choice as long as it's weather appropriate (and even that is negotiable - it's 55 here today and she left the house wearing shorts - although she did have a sweatshirt with her. I turned on the weather channel and said 'why don't you find out the weather today'. She has the info..... her choice and her consequence).
IMO her body is hers. clothes, hair, nails - at 8 she should be learning how to make decisions about her body and how what she does with it affects how she feels about herself and how she is treated (ie my daughter will be laughed at during school for wearing shorts today. She probably will think about doing it again). You provide reason and thought process. Sure you can paint your nails black, why do you want to do that? How will that make you feel? Do you think that will impact how others treat you and if so, will that be in a good way or a bad way and does that even matter? etc etc etc.
Also, if you start to ask her questions - do you need it or want it? She will start to learn the difference. Right now she doesn't understand the difference, so you will have to teach her the difference.
I would also find her opportunities to 'earn' money. My daughter cleans my bathroom (yay me!!!!). And I mean clean... tub, toilet, floors, mirrors, wipes off the toothbrush holder. for $5. She can do it once a week provided her other 'stuff' is done (her room, homework, piano practiced). She earns money that is HERS. If she has enough and decides that's what she wants to spend her money on... then she can buy the shirt and work for more money.
Sorry so long.... I guess I can relate to your post so I found I had a lot to say!
Just my $0.02
B.