I agree that a whip can be used like the extension of your arm. Lead with your right hand and if the horse stops, reach around with your left hand and give an encouraging tap.
Also, some horses will plant their feet and don't want to go forward. In that case, turn in a circle one way or another or even walk in figure 8's. If they won't go forward, I make them go in a circle or even back up. If your horse starts backing up, just make him keep backing up until he gets tired of it.
The most important thing is not to lose your patience. I think horses like to see you lose your cool. Just keep on making circles with the determination that you'll circle for hours if you have to.
Pretty soon a horse will figure out it is easier to go forward with you than do all those crazy circles.