While the title of this article will certainly get your hopes up, I'll have to bring you down a notch right away: there is no guaranteed way to shake off a determined 'rupter. That being said, and now that you are thoroughly disappointed in me, I will tell you how to make your enemies' lives more difficult. This applies largely to PvP only.
Update: Protips from a Reader: A buddy of mine pointed out a few other tricks when trying to avoid being interrupted by rangers. He says to use the terrain to your advantage and hide behind buildings, hills, columns, anything to obstruct their line of sight. Also pay attention to the distance between yourself and the ranger; if you're just barely out of his range it will take him longer to get within range to 'rupt. Not only that, but just the flight time for his arrows can be the millisecond you need to get that spell cast successfully, so if all else fails just make sure you're not standing next to him!