Graham,
Lumber still absorbs and releases moisture even if it has been kiln dried. This absorption and release of moisture causes the wood to swell and shrink. For example, a cabinet that is built in an area of high humidity may shrink when brought into an area of lesser moisture. This expansion and contraction may cause the joints in the cabinet to come apart, drawers and doors to bind, and panels to crack.

To counter this natural effect it is important to properly store the lumber. Ideally, the lumber should be stored in conditions similar to where the final piece of furniture will be located. It is also important that the lumber be stacked properly. It is best to use a number of short shelves to stack four or five boards on each shelf. If you must stack more boards it is a good idea to put stickers, or thin strips of wood placed every 12", between every 4th or 5th layer of wood. Because the face of the top board on the stack is exposed to the open air, it may tend to loose more moisture and warp upward. To prevent this it is best to place bricks or weights on the top of it.

If you plan to build a piece for inside your home, a good place to store your lumber is in the same room. Behind a shelf or under a couch is an excellent place to let the lumber acclimate to your house’s climate. Lumber and furniture placed too close to a fireplace, radiator, or other heat source may dry out and crack.

Plywood is also susceptible to warping. It should be stored on its end against a wall. If you need to store the plywood flat it is best to place three or four 2"x4" boards under the sheets.
-Woodmaster