I went overkill bracing my Klipsch cabinets....
I had a pair of Tangent 5000s that had come unglued on the back panels and since I had all the drivers out I went to town by reglueing all the panel joints on the inside with the Elmer's urethane glue (another member mentioned, good stuff!). Then I glued in 90 degree cut wood triangular blocks on the top, bottom, & sides for corner bracing (several of them down all four of the inside corners). After that dried. I installed braces from the front panel to the rear panel (cut exactly, to fit tightly) and crossed them with braces going from side to side. The braces were glued to the panels and glued where they crossed each other. I did not use any nails, but I did put in one inch deck type screws just barely into the panels to hold the wood pieces in place until the glue set up. Once the glue was dry, I removed the screws.
I had to do one half of each speaker at a time while it was laying on its side so that the glue would not run down the insides of the cabinet. I'd do a front and back corner seam, plus where the side panels met the top & bottom panels. Previous to the glueing and bracing the cabinets sounded like a drum when you'd slap the sides, but after the reinforcing had set up, all you'd hear was your hand hitting the surface of the cabinet.....no more hollow sound. I think the semi-flexibility of the urethane glue helped tone things down a hair also.
I even went beyond that and glued & stapled foam carpet padding on all sides of the cabinet (except the front) and the cross braces. The speakers did sound great after I got them reassembled and had plenty of bass, but no rattling back panel. I'm thinking of doing the same to my KLF-20s, but haven't gotten to it yet.