I've always been partial to the simplicity of a plain white window mat (or perhaps neutral grey for color work) in a classic Neilsen Gallery frame in either silver or black. Recently had some prints on temporary display for the Hamilton (Ontario) Art Crawl that were simply mounted to thick foamcore and trimmed borderless, the package then fastened to a blue fabric display-booth wall with industrial Velcro. While I'm go with a more substantial mat and frame for permanent display, for a temporary show I think this worked quite well.