Take one highstreet dress (Reiss "Shola" Dress at £175):-
See Kate Middleton wearing the dress:-
and voila.................a highstreet sell out in minutes!
I personally wouldn't wear this dress but it does look perfect on Kate.
It's amazing the influence that someone in the public eye has on us all. I myself have purchased things seen on some celebs. Take the Bright Blue J Brand Jeans, I first saw them on Kim Kardashian and thought they looked stunning on her so got a pair myself!
It's nice to get some of your influences from others although try to add your own stamp into the mix and make the look your own, after all it would be a boring world if we all looked the same and we are all unique!