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.