Don't Overbrush when trimming a wall - Overbrushing or "Roping" occurs and you will see lines. wait for the paint to completely dry before doing the second coat.
Get a grip! but not too hard! if you hold the brush too hard it will be a long day and your hand will get sore fast, dont forget to use your arm and wrist too.
when painting the top of a wall where it meets the ceiling I like to first do a long line of paint a little below the edge, dip the brush and in a smooth action complete the area. use long strokes and you should end up with a nice strait line.