This method saved the day Friday night while working on my first "paying customer" wedding cake.
Let the buttercream crust then use a clean, dry Viva papertowel to smooth the top and sides of the cake. Most methods say to use a fondant smoother, but it was 10:30 at night and all I had were a Viva paper towel, my hands, and a flat glass that I used to smooth with. It made the buttercream look like fondant. My customer was thrilled b/c she was worried that the buttercream would look too rough and she wanted a fondant look (I'm a buttercream only girl).