When your brush is loaded with paint, it's easy to create runs by applying too much paint in corners or along trim. To avoid that, start brushing about 1/2 inch away from the cut-in area to apply the paint. As the brush unloads, move over and slowly drag the brush along the trim or corner. Let the bristles gently push the paint against the cut-in area where the walls meet. You may have to do this a couple of times to get complete coverage, but it'll avoid excess paint along woodwork and in corners.