Summary

The worst nightmare of ABS printing is warping. These disks almost reduce completely warping.

Instructions

Add a disk at each corner of the 3D object you want to print at the bed level. Most of the time, a 1 cm disks does the job. More details are available at http://www.ctheroux.com/2013/02/preventing-warping-when-printing-3d-abs-objects/.

The best way to use the anti-warping disk with Slic3r is:

Add the anti-warping disks you need on the platter.
Add the object you want to print on the platter.
Align properly the object you want to print with the anti-warping disks.
Save the platter as an STL file. Too much plastic will be extruded if you save the platter as a GCode file right away.
Empty the platter.
Load the saved STL file.
Save the object as a GCode file.
Print the GCode file.