A drywall knife make a good, easy to use paint shield when painting adjoining surfaces with different colored paint. Simply hold the knife firmly against one surface while painting the adjacent surface. Then remove the drywall knife, wipe off any paint on it with a damp cloth, position it further down, and repeat.
Since drywall knives come in various lengths, smaller knives work great for painting around the panes of glass on window frames. Watch this video to find out more.