I also added 30" deep storage shelves on either side of our basement fire place. Here is a before/after of the basement.
Where did all the junk go? On to these big shelves, of course
Here is how they were made: 2"x3"s hung from the rafters define the frame and then two shelves with 2"x3" cross-bracing hold the plywood tops.
Finally, I installed shelves inside the fireplace to hold our house paint. Here is a before/after set of pictures. The shelves were recycled from old 2"xWides" that the carpenters used to install the windows.