You can remove brush marks by using fine sandpaper to sand away brush marks, ensuring that you do so evenly. To avoid brush marks, you can apply a thinner coat, as thicker coats tend to produce brush marks. If your work left behind some undesired marks from the brush, you can also get rid of them by lightly sanding them over the surface. Alternatively, you can try to apply the polyurethane in thinner layers as brush marks are left more easily with thick layers. Patience is key with this.