On square cakes. My corners are always somewhat rounded. Is there a technique for making them sharp?

This is something I struggle with as well, but CakeCentral has a good article on it here:
http://www.cakecentral.com/article51-How-To-Frost-a-Square-Cake.html

Oops....I forgot to say "when working with fondant" How do you get sharp corners?

Heh...whoops! I've only made one square fondant cake, so the best suggestion I have is to keep pulling down gently on the bottom edge of the fondant while you use your other hand to contour the fondant to the corner of the cake until it's nice and tight. It will never look as perfectly square as buttercream, though.

i don't mean to hijack the thread, but I am looking at bkeiths article right now practice and I seem to be confused when it says build up and then pull to the left (3rd step?)

I flatened the sides and got a triangle on the corners. After pulling the cornersthin, I was able to cut off the triangle and smooth it down to form the square edge for the corner. If you want, the next time I do a square cake corner, I can take pictures (step by step) to show the process.