Seems to be a case of 'lead from the front', 'manage from behind'.Explain what's not happening, what needs to happen, what will happen if things don't change, but also acknowledge the work you are satisfied with. When it's time to reevaluate, even if the improvement is small, acknowledge the improvement as positive, but encourage further improvement. Offer incentives to keep the improvement going if it can be applied to the situation. Have something set in place to use as a consequence for those not meeting expectations, but always start with a verbal or written warning (unless it is something serious that requires more than a warning).
At least, that's how I would do it.