Get the horse into a smaller area, because it's easier to keep pressure on them.
Keep them moving, because horses don't like "feeling the pressure". You don't need to run them (they'll run if they feel like it), but even in a 1/2 acre you can keep moving towards them and keep them moving. They'll get tired of it after awhile. If they stop and act as if they'll let you approach, but then take off again just keep them going. I've "pushed" horses even after they were willing to cooperate if they had a habbit of "acting" like they were going to be good.
Horses have excellent memories and they learn what gets them what they want and what they don't want (they'll also remember if they win and how they won). In this case the objective is for them to learn that allowing you to handle them keeps them from getting annoyingly pushed all over the pasture. And after being pushed around they'll see not getting pushed as rewarding, but feel free to praise and reward them when they finally do the desired behavior (they remember that too).
Keep in mind that, like people, every horse has it's own personality and figures things out at different rates (and often in different ways). I've had horses that it only took about 10 min and they were good from then on. I had one that took the better part of a week before she figured out that this routine would continue until she cooperated.