You aren't starting with a super sharp surface, your buttercream. You have to have that super sharp and finished already before you even think about putting fondant on. The thing that works best for me is to pop it in the fridge until it's super hard, then put the fondant on. Use two fondant smoothers at once. Pushing and smoothing with one in each hand, and towards each other really helps get a sharp finish.
I use SMBC, a meringue BC, so when it gets cold, it's hard...perfect for the type of finsh you are working with.
If you can, go get the Planet Cake book....they instruct you how to do this with ganache which is really helpful as well. I use it sometimes, but generally, I am all SMBC.