Also, make sure your cake is dense enough to support the weight of the fondant. Cakes should be completely cooled, stacked and iced together. Then chill for a few minutes before covering. Cake and buttercream will hold up better.
On Sharon Zambito's dvd, Flawless Fondant, she shows you how to the get sharp edges on fondant and she uses two of PME Easy fondant smoothers, like obesa said.
100g or 3.5oz or one third of a cup apricot jam whisked with 100ml or 3.5 fluid oz of boiling water until smooth. Remove any lumps and if you want you can stir in two teaspoons of orange liqueur for a bit more flavour.
According to the Planet Cake book, the big trick to getting the cake so flat with such sharp edges is partly due to the smoothers but also taking the time to get the ganache layer VERY VERY smooth with nice sharp edges with scrapers and hot knifes/spatulas before the fondant goes on.
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