We store sugar in a ceramic canister, and we get some clumping. Restaurants in humid areas that will often add grains of a rice to a salt shaker to stop clumping. (The rice acts as a desiccant and absorbs moisture, keeping it away from the salt; I believe the salt also acts as a preservative for the rice.) Is there an equivalent common household item that will keep moisture away from the sugar, or is getting a better canister really the only way?
Another solution is to save the little silica dessicant packages that come in many prescription drug packages. They are blue when they are fresh, and turn pink as they absorb moisture. Once pink they can be dried out by gently heating in the oven for a few minutes (10-15 mins at 200F or so). Place in the bottom of the sugar jar and check on them every few weeks or months, depending on the ambient humidity in your region.
I use a specialty ceramic stone to keep my brown sugar from clumping.
I found this product page http://www.cooksquarters.com/19386341.htm that has a pretty thorough description of a "Sugar Saver" that can be soaked to keep sugar moist, or dried in an oven to keep spices dry. Depending on what your sugar needs, you can probably use something like this.
Gilladian is very correct about using silica dessicant packages except for these issues...
The Silica that turns pink when it has absorbed moisture is known as "indcating silica gel" and has a chemical in it that is NOT FOOD SAFE! (you can find out about it on line).
The silica packets found in prescription drugs often contain other ingredients like carbon (which absorbs odors), and some others I won't go into.
The silica packets found in prescription drugs have been mostly 'used up' by keeping the drugs fresh so the drug companies can get a longer shelf life.
That said, I found a wonderful product called Dry Spice that is especially made to put in with sugar, spices, almost everything. It is the perfect size and shape to fit in any size spice jar or whatever, is 100% natural, and perfectly food safe...and wors REALLY GOOD! Check out www.dryspice.com and you will see what I mean.
Readers beware: The user who posted this has an undisclosed affiliation with the above product. Food-grade silica gel is readily available on the market and readers are encouraged to do their own research.