Sorry to drag this really old thread up again, but I was fortunate recently to smuggle a wasabi root home from Japan a few months ago (it only cost me $16 at one of the food halls) and after I had my fill of putting the fresh ground root in all sorts of other foods, I realized that I needed a good way to preserve the remainder.
My solution? I cut the rest into chunks, mixed in some salt and canola and dig a quick chop in a mini food processor. Then I spooned the mixture into one of those little frozen garlic trays you get at Traders that looks like a mini-ice cube tray-- and I froze it.
Now I can pop little cubes out when I need them. While the frozen isn't quite as awesome as the fresh, it still blows the doors off the dried wasabi you get from Penzeys or the fake crap you get from most sushi joints.