I wouldn't say Kaplan is the ONLY way. However, I would be cautious with Princeton. A friend of mine took Princeton a few years ago and swears by it. Last year when I was researching prep materials, I contacted Princeton. The response from an "honest" princeton employee, was not to bother, they were getting out of usmle prep, "they didn't make any money on it". Each of us have to prep according to our educational background, strengths and weaknesses... one-size-fits-all approach will not work. This site, however, has provided a great deal of info.
I will provide a disclaimer... I failed the first time by 1 point. I am rescheduled for thursday. 😱 I used Kaplan exclusively the first time, but don't blame them. I didn't feel well that day, physically or mentally. This time around I have reviewed my Kaplan books and more questions. While no one likes advice from a loser, I feel you can learn from your mistakes.