I have got myself a Purdy Nylox Sprig flat brush, and I painting the door trim and using semi-gloss. Is it the proper way to paint in one stroke and back brush once or twice?
Two things that will make a big difference;
1) Condition your paint. Paint conditioners will help the paint flow out on the surface and be less sticky. Use Floetrol for water based paint and Penitrol for oil base paint.
2) Quickly apply the paint to an area then go over it once to lay it off, twice max. The more you play with the paint the more likely there will be prominent brush marks.