Hops usually occur because we ignore the radial trueness in our effort to attain lateral trueness. Generally, I can flatten a hop by tightening the spokes on both sides in the high section. If it's a wheel I'm building myself, I'm usually truing out the hop before the wheel gets too tight. If it's an old wheel, I'll carefully loosen the whole wheel by a half-turn and start over, paying more attention to the hop.