After upgrading by Printrbot LC with longer (400mm) 8mm smooth rods and M5 threaded rod leadscrews, and installing z-axis stabilizers for the bottom of the rod (http://www.thingiverse.com/thing:46082), I still wanted more stiffness.
Other clamps on Thingiverse were not to my satisfaction. Some coupled the smooth rods together by clamping to a rod (or rods) that were not in the same vertical plane as the ones used for motion, creating unnecessary stress at the points where the rods are fastened. Others required a very specific length of rod, or were too bulky or heavy for my liking.
For an open-frame FDM printer, like the Printrbot LC or LC+ (new and old), these allow you to use your old 8 or 12mm smooth rod or 5/16" threaded rod (the threaded rod would almost be too short, but should work) to couple your z-axis threaded rods together for increased stability. As you will lose approximately 1in of travel in the z-axis, these will work best if you have already extended your z-axis smooth and threaded rods (this can be done quite cheaply, for a total of $20-$30 in parts from Amazon).
These are designed to clamp to a rod of the same size as your smooth rods (8mm for Printrbot LC or 12mm for the LC+). Threaded rod or smooth rod can be used as long as it is close enough in size to be clamped to.
Print 4 copies of this part (two for each rod), and fasten them using a total of eight 3/4" 6-32 machine screws and nuts (M4 could also be used). Be sure to check that your smooth rods are parallel (and not \ /) before tightening the stabilizers. Enjoy!