Originally Posted by zep33
I had no idea what I was doing when I built it - First time I ever built anything other than a plastic model airplane
One way to add stiffeners to your shelves is by adding a back to the unit. This can be thin paneling, ⅛" plastic or fiberglass wall board, etc. Items are readily available from Low's or Home Depot. You can use #4 finishing nails, small screws, tacks, etc. to attach the back to the unit.
A nylon string stretched across the unit will assist in making sure the individual shelves are straight before securing the back to the unit. Also, make sure the unit is square (3, 4, 5 right triangle) before securing any fasteners to the back. After squaring, add a few fasteners to the perimeter of the unit. Next add one fastener (nail, tack, screw) in the center of each shelf and one fastener on each shelf end. Then you can lay it face down to add additional fasteners.