I cannot say much about the best way to store them, because I, too, am a beginner. I do, however, have 8 or 9 tins that have all been opened, some for several months. I have left them in their original containers and none of them have lost their scent or flavor.
The ones that has been open the longest are American scotch snuffs, so I don't know if that has anything to do with it.
By the way, that Viking Dark is a BEAST of a snuff! Took me a couple go arounds to really appreciate and enjoy it. Also, if you like menthol feeling and scent in your nasal passages, you might want to think about trying Red Bull. Gary Alan Richards sent it to me and it's pretty amazing. It's like a burst of clear nasal passages awesomeness. :-)