When I'm covering a square/rectangle cake with fondant I can't get a sharp edge and the cake looks more like a pillow than a box. How can you keep this edge?

I haven't even attempted covering a square cake so I'm curoius to hear the tips too

Here's some tips for sharp corners:
make sure your fondant is not too thick.
start with sharp corners when you ice the cake, you'll have better luck keeping them sharp.
Chill your cake before covering it

use two fondant smoothers like this http://www.youtube.com/watch?v=nbU5O1YRjBI&feature=related

Ice with ganache under your fondant. It stays very firm and holds a very sharp edge. http://sugarsweetcakesandtreats.blogspot.com/2010/05/covering-cake-in-ganache.html

practice practice practice

One trick I've learned is to bake your cake an inch bigger than you need. Then fill, stack and chill the cake really well. Then take a knife and trim each side of the cake so the sides are perfectly straight. Then crumb coat and chill thoroughly again before covering in fondant.

fter you have your fondant on you can run your fingers along the edges of the cake, one on the top and one on the side, effectively "pinching" the fondant into a sharp edge. You can also cover te cake with modeling chocolate panels (or put them on the cake before the fondant). HTH!