Many people who built the beta version of the Printrbot Simple have experienced a sag in the Y axis, causing the print bed to be way off level. This upgrade takes care of that completely on my printer, and hopefully on your printer as well.
It fixes the problem in two ways. First, it provides a very secure, curved slot for the bearings, which helps keep them lined up. And second, it has a built-in 1.5 degree slope to counteract the sag.
There are two parts: a wedge and the bearing block. Print out two copies of each (one for the top bearings and one for the bottom).
Remove the zip ties just from the top and place the first bearing block into the laser-cut slot. It can go in only one way because there is a cut-out for the screw used to hold the laser-cut part that holds the Z nut. Next place the wedge in from the back and push it firmly in place. Finally, add the zip ties, but don't tighten them completey yet.
Now do the same for the bottom bearings. Once you have the blocks and wedges installed for both the top and bottom, tighten and trip the zip ties.
Re-level your print bed and you should be all set to go.
A special thanks to twistedmedium on PrintrbotTalk who had a much simpler idea that I ran with. Here is the Fixing Sagging Y thread on PrintbotTalk.
I printed the bearing block using Kisslicer with support material, as you can see in some of the photos. I like Kisslicer because the support material is very easy to remove.