There are a few key things a president needs to protect his power. The first is intelligence, especially details of process and policy. Without these, he will not understand the stakes of any choice for his futures choices.
Beyond this, he needs time to see and make decisions. The modern presidency beset by deadlines; to have the chance to preserve his ability to make choices a president needs to get ahead of them, to give himself a buffer. Setting personal deadlines ahead of the one's imposed by necessity is one way to create such a buffer.
Roosevelt fostered competition to gather intelligence and expose the decisions he had to make.
Eisenhower used his staff to shield himself from conflicts of information and perspective.
qualities that enabled Roosevelt help himself to grasp the power stakes in his decisions:
institutional understanding, enjoyment of political power, ambition. confidence
Eisenhower lacked all of these. His Army career did not train him for the methods of political power. He disliked political gamesmanship. He sought national unity rather than any substantive goals. His self-confidence depended upon his self-image as a statesman who stayed above the fray, and thus failed to help him assess the power stakes in his choices.
Eisenhower's focus was usually too broad to give effective direction to his use of power. When he did take personal interest in policy details, however, as in his balanced-budget crusade during his last two years, he tended to lose track of any broader aims.
Truman was open to information from many sources and was eager to make decisions. He was accessible and read documents avidly, but he focused on immediate decisions and their circumstances rather than trying to put together disparate information into a larger context. By temperament he was a judge rather than a chief of intelligence.
Truman sense of power was shaped by his experience -- and sometimes his lack of it. As a former Senator and a party organization man, he placed a value on the prerogatives of Congress and loyalty to subordinates which tended to obscure for him how these restricted his own influence (although he learned largely to dispense with the former). Having never headed a bureaucracy, he had little feel for how his initiatives could be obstructed.
Never having had the ambition or expectation of being president, Truman was unusually sensitive to the difference between the office and its holder. Truman's sense of confidence was tied to being able to see himself perform the role of president, which he saw as initiating and deciding. Since so much was wrapped up for him in playing the role of president, however, he was reluctant to upstage or interfere with subordinates lest he appear to be letting personal preferences get in the way of policy, and this, as was the case with MacArthur, could hinder his sense for power.
Truman had a strong convictions about what to do as president -- he saw himself as the heir and protector of the New Deal and internationalism and the mid-century Democratic coalition. He saw it as his duty to sustain the legacy. While the focus this gave him was sometimes effective, it often led him to strong fixed positions whether or not this put him in the best situation to influence policy.
Overall assessment: Truman and, especially, FDR made effective use of presidential power; Eisenhower, because he was a political amateur, did not.