a commanding officer gives a soldier an order that he knows the soldier would rather not obey. according to the findings of the milgram obedience experiments, what should the officer do to increase the likelihood of obedience

I believe the answer is: stay in the presence of the soldier until the order has been obeyed

According to millgram experience, an individual will much more likely to do something if it's dictated in the presence of someone that he/she recognized as an authority. In the scenario above, we could make the assumption that the presence of the officer would most likely make the soldier feel pressured to follow the order.