It's common to believe, when you listen to great speakers at work, that certain people are simply born with the talent to speak well, and therefore no amount of training or practicing is going to transform you into a great speaker, no matter how hard you try. And while it's true that both John Kennedy and Bill Clinton have what most people don't - charisma - their ability to speak as they do was not part of what they were born with.
Bill Clinton was not always a great speaker. He was guilty of several bad practices, one being common to the political class: Clinton actually thought people wanted to hear elected officials go on at the mouth for one, two, or even three hours. The truth is you rarely hear someone say, "That was a great presentation - I only wished he would have droned on for another hour or so". And FDR is famous for his advice to public speakers: Be sincere; be brief; be seated!
Brevity is the soul of wit, but it's also the heart of a great talk. President Ronald Reagan, the person for whom the name "The Great Communicator" was coined, had a strict limit of 45 minutes, but preferred to stay under 25 whenever possible.