Try grinding them up with a little oil a la pesto. Then freeze. Holds up better than just freezing the leaves whole. They do lose their brilliant flavor but I guess it's better than no curry leaves at all.
I have also frozen my extra curry leaves with few ill effects. I just figured 'what the heck, they'll just wind up in the garbage either way...'. The only thing that I'd change is that I'll be quicker to get them into the freezer next time.