I picked up this tip from LightroomKillertips.com this morning and just had to share it with everyone.

If you are using the new adjustment tool in Lightroom 2.0, this is going to save you lots of time in getting your adjustments painted in properly. If you hit O (oh, not zero), while you are in the adjustment tool you can turn on the overlay and paint with it still turned on. Hit O again to turn it off. You can also use Shift-O to toggle the overlay colors. A second tip is to use the Alt key to “un-paint” an area. This video shows how these shortcuts work.