I'd try putting the slightly opened bag of sugar in an airtight canister or plastic along with a wet paper towel. That has worked very well for my brown sugar.
By
02/09/2010
Just keep it in an airtight container, but do NOT put a damp paper towel in with it. While that is great for brown sugar it will only cause powdered sugar to clump into one hard brick. This is the voice of experience!
By
02/07/2010
I store all sugar in airtight plastic bags or a can, good luck.