http://cakesdecor.com/sugarpixy/blog/169 Here is one right here on cakesdecor. It takes Lots of Practice! What method you use also depends on the type of buttercream you use. American, SMBC, IMBC, ect. I use American buttercream and I use half butter half hi ratio shortening mixture. So I use the Viva Paper towel and Hot knife methods. But here is a method used for at least half butter or more recipes UPSIDE DOWN METHOD http://cakecentral.com/tutorial/upside-down-icing-technique-for-perfectly-smooth-icing This works really well.
Another hint is that after you have your buttercream pretty smooth. (american buttercream) You can take Copy paper and a fondant smoother this will make it perfectly smooth!.