I've never done it myself, but I've seen people answer your question in 2 ways:
1) Put a layer of wax paper or cling wrap behind the ribbon before attaching it to the cake
2) Put some Crisco on you thumb and index finger and pull the ribbon through them to lightly grease the ribbon uniformly before attaching it to the cake.
That's what I did... I cut parchment paper in the exact width of the ribbon I was using and it worked...only thing is, I was using SMBC and when the buttercream got too soft, it still got stuck to the paper, but no grease on the ribbon.
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