Well it took something like 150-200 years to fully assimilate the Norse into Christian Europe, even working from the top-down (i.e. royalty before commonfolk), and during their conquests of Britain the Vikings did 'convert' many of their new subjects to Paganism. I think the easiest way to 'save' Norse Paganism would be to have the Norse kings and leaders more fervently opposed to Christianity as a whole, or to have Christian missionaries go about things the wrong way.
I remember reading that in many instances, Jesus was protrayed as a vicious warrior in order to make him more appealing to the Norse (and Germans earlier). Perhaps one way would be to have the earlier missionaries preach Christianity as a religion of peace, or have a Pope opposed to the change of imagery of Jesus, and thus it simply does not appeal to the Norse when their own gods fit their image of what it is to be human much more appropriately. Of course, this is probably too vague a POD.
Also, having the Norse 'expel' Christians from their territories, in the same way many East Asian countries later did, could work. I don't think 'traditional values' are as important to the Norse, who readily converted in order to rule over different people, as it was to the Japanese.