Painting a straight line at the intersection of walls and ceiling is what separates the pros from the hacks. To do it, start with a top-quality 2½-inch angled synthetic sash brush of medium stiffness. "Dip the brush about halfway and tap it on the side of the bucket. Don't squeegee the brush or you'll end up with too little paint," says Rich O'Neil of Masterwork Painting and Restoration in Bedford, Massachusetts. Holding the brush parallel to where you need to cut in, make a preliminary "glide strip" about ½ inch from the intersection. Working in long steady strokes, fill in the area between the strip and the intersection. "The brush will move smoothly and you'll have better control," says O'Neil. Be sure to feather your strokes to prevent paint buildup that could show through the roll coat.