The Secretary of Defense is not a super General or Admiral. His task is to exercise civilian control over the Department for the Commander-in-Chief and the country.
Don't necessarily avoid sharp edges. Occasionally they are necessary to leadership.
Don't be a bottleneck. If a matter is not a decision for the President, or you delegate it. Force responsibility down and out. Find problem areas add structure and delegate. The pressure is to do the reverse. Resist it.
Be precise. A lack of precision is dangerous when the margin of error is small.