Clean and dry, drop pressure by half, bag each assembly and get as much air
out as possible(many tire stores will actually give you bags for this
purpose) and then seal the bags with duct tape.  Keep them away from heat,
like next to the hot water heater or furnace, or direct sunlight..  Keep
them away from electric motors (like the blower motor in the furnace) as the
ozone created by electric motors is harmful to tires.
Store on their side, stack no more than 3 ft high.
With the tires bagged, the pallets aren't needed fro air circulation, and I
would rather have the sidewalls evenly supported.
These recommendations can be found on Goodyear, firestone, Hoosier, and
other manufacturers web sites.

Little snow tire tidbit of trivia, in most European countries, such as
Germany if you use snow tires you are required to have them on all four
wheels, not just the drives.  Failure to do so can result in a ticket and
being found at fault in an accident.
Whitelightning

There is no "official" way, unless you are extremely anal
about these things.  It's only for 4-5 months...just throw
them in the basement and forget about them.  It sounds
like there are some folks that are "extremely anal" about
these things.

Ian