When lunging, imagine a pizza slice or traingle. The horse is the crust or base, you are the point, and your arms are the sides. The lungeline and whip are extensions of your arms, no more. Lets say the horse is going left. Your left arm needs to be a straight line from the horses head, down the lunge line to your shoulder. Your body should be angled slightly behind the horses movement to "push" him on. The whip is in your right hand to "close" the triangle. The whip should not be flailing around, but pushing towards the horses butt to drive the horse forwards. If you need to flick it, flick it towards the horses butt in a sideways motion, again pushing the horse forwards.
If the horse stops and stares at you, shorten the line and keep turning your right shoulder (going left) so it is angled towards his but. Don't smack him with the whip, but feel free to push it up behind him. If neccesary, pratice with a lead rope and just make him walk in a small circle. Then you can gradually expand the circle.
Good luck! I hope I helped, it's difficult to explain