Use you hands and arms to a purpose, don't just swing them wildly. Don't bounce from side to side like a boxer. There best way to improve this is to bring it into your conscious. Start thinking about your running posture then focus on correcting one thing at a time. Keep at it.
Bad habits are hard to break.
When you are correcting a habit make sure you run with the new habit in all runs until it becomes ingrained. Practice doesn't make perfect, perfect practice makes perfect.