The Entitlement Issue

I've noticed Jane demonstrating a new attitude lately that really gets under my skin--the whole "If I want it, I will have it" mentality. Now if you are grown and have worked to be able to provide yourself with everything you want, kudos to you. But an 11 year old--not so much. Examples:

"I need heart socks and headbands and earrings to wear every day for February. Think you can make that happen?"

"You have to do my nails with blah blah color and blah blah pattern for my school dance Friday."

"Uh, no....I'm not wearing this homemade toga for Greekfest (that you worked all day on and I forgot to tell you about until the day before.) You need to go to Party City and get me a real costume."

Yes, she is a very privileged child and has always received pretty much everything she wants. The difference is that now she is starting to demand it, and believes she is entitled to her every whim. I know she's a little old to start breaking her of this now, but what's been done in the past is past. We've got to address this attitude now. And it is very hard in a stepfamily to not give our child what they want, because in the back of our heads, we know if we say "No", they will just go demand it of their other set of parents. That won't teach them any lesson at all. Several times Jane has asked for something, and I have purchased whatever it is, and saved it for birthday, Christmas, Valentine's...whatever is next, only to find she went ahead and already got it out of her mom. So we have to approach this carefully. Maybe we should let her know she is going to get it if... and put conditions or chores on as a way of earning it instead of just being handed everything on a silver platter. She still may run to her mom for it, but maybe there will be less chance of that if she knows we are planning on it once she has earned it. I don't know. How do you other stepfamilies handle this entitlement attitude?