The best neck harness is the Ironmind model-
www.ironmind.com. I've also heard good things about the Spud Inc. one
www.elitefts.com sells.
They are expensive, but I have absolute confidence in the Ironmind one. I've had it for 6+ years, and it's still like new, and very comfortable. Neck harnesses tend to get a lot of wear and tear, so buy a nylon one that you can wash. I've seen a few leather ones fail do to rot after a few years.
I train neck 1 or 2x a week depending on whether I'm doing high pulls, etc...... Neck doesn't require much work 3-4 sets.
