Thanks all for your replies

(amazedmom, we have had the same marker problem in the past too!).
It is reassuring to know it's not unusual. I will definitely give the preparing in advance thing ago - that's a great idea, I've tried it a bit in the past but never to the same extent and it always petered out. I re-read my paragraph about limiting choices and realised I hadn't really said what I'd meant to. We do currently limit her choices (2 options for breakfast, clothes etc), but had tried really limiting them (I got a lot of responses to this question elsewhere suggesting that I simply remove all choices for dd as other parents had found it resulted in a lot less stress for their kids - I was skeptical about this - from my perspective this didn't seem helpful long term. Even in the short term we tried it, it was a disaster - particularly because I didn't really believe in it).
I can see how pre-planning could also reduce my need to use a time limit too (which are quite long - for example I might say "we're going to look at the magazines for 10 minutes, but then we'll have to go. If you haven't chosen one by then we wont buy one now, but you can continue to have a think about it and we can come back another time". Which all sounds very straightforward when I write it down - but never is in practice because then it adds the 'choice' of making a decision then or waiting to make the best choice and missing out now. So it always ends with a very upset girl!) Hmm... mind is whizzing with the possibilities of pre-planning and how we could apply that (Lori - I love that your son checks out prices and reviews - dd loves a bit of research, so perhaps I could get her having a look at her options online before we go and buy a gift/book/toy etc) and of course, setting up clothes and food options in advance is good common sense, which I shall apply

I guess other than that we'll just continue to work with her to help her understand that ultimately what her decisions aren't life or death and hope she grows out of it (or should I say into it - I like that you mentioned that you mentioned the positives of not being impulsive Lori, I will think of that when I am feeling frustrated!).
Thanks all.