How do I get fondant onto a square cake? I can never get the corners neat... What do I do with the excess fondant??? 
first, chill your cake well. ice, chill again. roll out the fondant 1/4" thick and larger than your cakes width and sides, in a square shape. place it and smooth the top, fluff the edges outward. then cup the corners in your hands one at a time, then smooth the sides. trim with a pizza roller.
Hard to explain in words alone, but you gently pull it down and trim the excess. My Wilton instructor showed my how... she did it very well. I think it just takes a lot of practice. We covered one of my square cakes in fondant... rolled it thicker than I would have imagined necessary, but it made it easier to pull (very carefully) out and then pressed it down. We placed a boarder around the base to hide a few imperfections.
def roll the fondant a little thicker than normal, smooth the top, then the corners, pressing the excess lightly towards the middle, then smooth the centres downwards and cut off the excess. A set of fondant smoothers is a godsend for this task!
HTH and good luck
Here's a pictoral:
http://www.pastrywiz.com/wedding/wedding19.htm
Additional info on covering square cake w/fondant:
(more tutorials)
http://www.cakecentral.com/cake-decorating-ftopict-55290-.html
http://www.cakecentral.com/cake-decorating-ftopict-54484-.html
http://forum.cakecentral.com/cake-decorating-ftopict-55363-.html
HTH
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