For myself, whenever I get postcards or for that matter business cards made up, I always finish them with what is called a cello glaze finish. This is a very light plastic coating, which I usually have done in a satin finish. The quality this gives is exceptional for only a small investment. I also request that this is done on the cover of any catalogues that are produced for my exhibitions.
Sometimes you may wonder whether it’s worth the expense especially if you are producing 1000 or more postcards, in my experience it’s worth the expense simply because it becomes a postcard that people are more likely to hold onto or pass on.