Ok, I'll tell you my first idea, not sure if it'll work, but it might be worth a try???

Could you try giving her an 'x' number of things she can ask for (and receive) each day? Say you were going somewhere tomorrow and you gave her 3 'tokens' (so she doesn't forget how many she has left) and she can ask to do/have 3 things that hasn't already been promised to her (eg you may have already said you would take her to the park, so it doesn't count). She would need to understand that she wouldn't get every thing she asked for (esp if it cost money for example) and denied things don't count (although if she kept arguing or winging about these things you may take a token away). once she has used all tokens that's it, no more asking.....eventually she might learn to only ask for things she really wants????