Apply painter's tape to the surface to be painted, securing only 1 or 2 inches of tape at a time to ensure a straight line. If you are painting a wall, press the tape as close as possible to the edge of the corner or molding line.
Press the edge of the tape that faces the surface you will not paint to remove any ripples or puckers where paint could seep through. Use your hands or a putty knife to press firmly.
Paint over the edges of where the painter's tape with clear paint. This coat will seal any gaps in the tape, and if any paint leaks through, it will be the clear paint which will be invisible on the wall.
Paint the surface, using care not to paint on the wrong side of the tape.
Remove the tape after the paint has dried for several hours. Pull the tape away from the wall slowly without pulling at an angle. Peel the tape back over itself to avoid tearing the tape or removing fresh paint from the surface.