Shawn L. Samaniego sends us a tip on unselecting objects from your selection set in AutoCAD.

"When selecting items in a drawing, it's common to select an unwanted item by mistake. Without exiting the command you are currently in, type R (for Remove) on the Command line, and select the unwanted items. To return to the command selection process, type A (for Add); then you can continue to make further selections."
 
Notes from Cadalyst Tip Patrol: Picking the right objects in your selection set can be tricky, especially if there are many objects that overlap, or if it is a busy area of your drawing. There are two ways to "unselect" objects; the first is detailed above. If you don't want to type, you can hold the Shift key down instead and select the objects you want to remove. AutoCAD 2011 includes a new feature that helps you pick the right object; it will list the objects near your mouse and let you select the one you want by choosing its name from the list.