When dippping the brush in the paint only load about one-third(1/3) of its bristle length. Tap the bristles against the side of the can. Do not dip deeper as it overloads the brush. Overloading also results in dragging the brush against the edge/lip of the paint can to remove the excess paint, which in turn may cause the bristles to start wearing down.
A 4" straight-edged brush is a perfect choice for cutting paint lines in corners and at ceilings. For woodwork you can use a 2" trim brush. Be sure to choose brushes with chiseled tips for painting corners.
When you are cutting in, cut in the edges using the narrow edge of the brush, pressing just enough to flex the bristles. Keep an eye on the paint edge, and paint with long, slow strokes. Always paint from a dry area into wet to avoid lap marks.
For large areas, apply the paint with 2 or 3 diagonal strokes. Distribute the paint evenly with horizontal strokes and hold the brush at a 45 degree angle, with just enough force to flex the bristles.
You can smooth the surface by using light strokes and lifting the brush from the surface at the end of each stroke. Draw the brush vertically from top to bottom.