Activate the Next-step button only when the user has responded to all mandatory fields and there aren’t any errors. Obviously, apply this trick only if you use inline validation with the real-time feedback (like described in trick #9). This will keep the user focused on the answers and will not let him to skip something important.
More importantly, the animated activation of the Next-step button will work as a visual anchor for the eyes. So the user’s focus will jump to the button at the right moment. And that’s exactly what we need. But to make this approach work properly, the distinction between disabled/enabled states should be very explicit and visually clear.