Light and lots of release of pressure and also fitting correctly.
Before I would switch to a mechanical hackamore I'd teach my horse to relax his jaw on cue. Look for information on flexions as taught by Phillipe Karl or Baucher.When a horse has learned this, it can be ridden on a very light contact and a sensitive rider can stop rooting before it starts. As you're riding, you ask the horse if it is still relaxed in the jaw. You can feel if it is. If it isn't, you immediately ask for flexions that relax the jaw. Problem solved before the horse gets frustrated and starts rooting on your hands. Frequently drop the reins and ride with no contact too. A horse that doesn't have constant contact gets less frustrated than one that is constantly held in :)
I ride in a mechanical hackamore because frankly my horse goes better in it than any bit I put in his mouth. But be careful, hackamores can be quite severe on the nose and poll and aren't really an appropriate choice for most horses/riders. I would suggest talking to a respected trainer, getting your horses teeth checked, and consider other bit options before I would go to a hackamore :)