When cutting in for painting projects, use an angled paintbrush and a paint bucket with no lip on the edge. This prevents paint from accumulating along the edge of the bucket, and gives you room to wipe excess paint from the brush. Fill the bucket with only a couple of inches of paint.
The grip of the paintbrush is also important when cutting in. Use a pencil grip, which allows for good control without tiring out your hands. Use the thumb and index finger to cradle the brush handle. Grip it near the bristles, just like you would if you were holding a pencil. Dip the brush about 1/2 inch into the paint and wipe off any excess before starting to paint.
When applying the brush to the trim or wall, create a wedge shape with the bristles. The sharpest end of the wedge creates the paint line. Begin moving the bristles along this line when cutting in. Do so slowly, allowing the bristles to naturally create a fan shape. Only the outer bristles are doing the painting.
Overlap the different sections in a subtle way to avoid marks left by the bristles of the paintbrush. It helps to practice the technique on cardboard before cutting in on a wall.