I would first clean the couches either professionally or if you have one with your own cleaner. I have a Little Green Machine that is for cleaning carpets and upholstery. They make special cleaner that is supposed to help with pet smells. After cleaning them, there are three different products you can try which can help further remove the smell. Febreeze is one which has already been suggested. There is a product that they sell at Sam's Club (and other places, I'm sure) called OdoBan. It works well, also. Additionally, Renuzit makes a product called Odor Neutralizer which comes in different scents. It works well, also.
Good luck.