I made and decorated a beautiful cake for a family friend, and it’s too late to do anything now. I decorated the cake on a cake board, and now there are greasy smears on it from the buttercream.  How do I prevent this in the future? It didn’t occur to me that there would be marks on the board from the buttercream. I am still new to this. I also feel bad because I have to give the cake to her like that. Advice? Thanks everyone. 

one way is to get some paper doilies a small size is ok -- cut a little stack of them in quarters, then cut off the points and after all is said & done you slide them under the border with a butter knife or spatula, the metal icing spatulas -- or you could slide them under the cake before you border it -- over lap them so it just makes a pretty easy peasy look --

or decorate your cake on a board the same size as the cake and affix it onto the food safe wrapped cake board you reserved for it -- wilton sells a nice foil -- wrapping paper is not food safe neither is florist's foil although ff is often used -- it's ok if you keep the cake on a food safe board-- you can affix the boards together with either a blob of icing or hot glue or a roll of double sided tape --

I alwasy cover the board like K8 said w/a food safe 'paper' (coverings) having the cake on a board the same size (or slightly smaller) ie: 9" cake on an 8" board.  Of course it's not always possible to find the board a bit smaller than the cake

I decorate and serve from the same cake board. I buy greaseproof boards, and once I finishing icing, I take my spatula and clean up the board. If I'm feeling fancy, I will decorate say a 6 inch cake on a 6 inch board, pop it in the fridge to chill, then glue that board to a decorated board. It is a pretty cake, but things happen. Everything is a learning experience, and don't be so hard on yourself!