If you make enough prophecies at least some will come true, especially if you make them vague enough. Take Jean Dixon: she predicted that a Democrat would win the 1960 US Presidential election and that he would be assassinated in office. Well, JFK won...but she'd predicted that he would NOT win the presidency.

She also predicted that Atlantis would rise in 1968, that the year 2000 would see the beginning of lasting world peace, that WW3 would begin in 1958, that a "Middle Eastern child" born on February 5, 1962 would unite all warring religions and sects, that there would be a holocaust in 1980, cancer would be cured in 1967, and that Richard Nixon would serve his country well.

Make enough predictions, make them sort of vague, get them hyped by the media...and you're a prophet!

(By the way, the odds of Kennedy being assassinated while in office are about the same for any president of the US: 7 to 3 that it won't happen. That's from professional odds makers and if you doubt it, go look it up yourself.)