I've seen people use ganache as a crumb coat and i know that that helps with making clean cornered edges as opposed to rounded, but if you don't want to use ganache, then how do you do it? I've searched for this topic, and un;ess i'm not looking good, i'm not finding how. That is my biggest problem with cakes... the more i tamper with it, the fondant starts to rip or it just doesn't take...
Check out this 3 part series on YouTube by InspiredbyMichelle:
It will show you all the steps from making ganache to covering in fondant.
For a buttercream cake, try the upside down method of icing a cake. If you then refrigerate the cake, I'm sure the Fondant would go on to have nice and crisp corners as well. HTH
Sorry...I just re-read your original post over and saw that you said you did not want to use ganache. The techniques in the videos are still helpful. jessicakesblog also had a really good video for the upside down technique http://jessicakesblog.blogspot.com/2011/06/video-tutorial-upside-down-frosting.html
I haven't tried that yet but it's on my list (personally I haven't yet mastered the crisp edges myself with buttercream but....someday).