I just finished three more of our new terrain boards (a desert/dried grass one, another hilly/grassy one and an urban one) - giving us a total of five. However, the one big problem facing us when we started this project - how to store the boards had yet to be solved.
Typically boards like these are stored in some sort of permanent rack system, usually horizontally. However in our case there were some extenuating circumstances - namely available space, time to build a rack and the ability to put more boards in the storage system as necessary. What I came up with sounded a little bizzare at first, but on paper it met all of our needs.
Essentially the terrain boards are stored vertically, with one edge resting on the floor. The other end is then held up by using tension from the rack. There is a little movement allowed at the top of the racks for sway, due to someone bumping the racks, etc., but not enough to allow the boards to move more than and inch or so.
Here are a couple of pictures of the racks. As you can see, there is plenty of room for additional boards as there is space between the boards that can be eliminated by moving the tension points and increasing the tension from above (more tension equals less sway, and thus less space between boards needed). The rack was fairly easy to construct, with a minimum of materials, and should still be able to be moved around if we need to shift things about.
In the end we are pretty pleased with the new boards and their storage system. Now I can turn my energies to some more unique boards for the shop - a beach board for amphibious landings, and exposed dungeon & hills board for fantasy battles and maybe even a snow board... 
Well, until next time...
Cheers!
Dan