ABS is notoriously difficult to get dialed in. I recommend folks to use PLA when they are first starting out. Once things get dialed in well, then go to ABS.
ABS shrinks as it cools. As your lower layers cool, they begin to shrink. Since the layers above it are still warm, they do not shrink as much yet. This causes curling on the lower layers.
The key is to get to the right printing temp, usually as low as possible to still get good prints.
The other key is to try to keep the whole interior of the print as warm as possible, usually with an enclosed print area.
You can also try helper disks - http://www.makerbot.com/blog/2013/04/19/keep-corners-flat-with-makerwares-helper-discs
Long flat areas and square corners shrink the most.