Here's a tip for frosting cakes to leave a clean edge on the bottom:
After wrapping your board (I just use wrapping paper), tear off 4 strips of waxed paper and place them around the edges of the board.
Then, place your cake on top, so each of the edges has about 1-2" of waxed paper underneath the cake.
Frost the top and sides of the cake. When you're finished just pull the waxed paper out from under it, leaving a clean bottom edge and no mess on your pretty paper!
It's best to pull opposite sides out at the same time so the cake doesn't slide with you.
(Grab the left and right papers and pull at the same time.)
You can go on and add a decorative icing border after you pull out the waxed paper, such as a shell border or rope border to anchor the cake, but at least you're starting with a clean edge!