First and foremost: practice, practice, practice.
But at the end of the day, the key to product marketing is storytelling, right? You should think of your career in the same way. Run through your resume and build a storyboard to help you articulate why you made each move and what you were trying to accomplish at the time. Treat yourself as the protagonist in this story - what challenges were you facing, what situations did you overcome, what activities led to you becoming the hero? You know the answers to these questions better than anyone else... So, start running through it - maybe you left your last company after less than a year - why did you leave? Was there an opportunity to learn something new in another area or aspect of your career that you hadn't done yet? Once you build your career story, then start practicing - tell your story to a friend, roommate, significant other and have them ask you questions throughout. This can help you prepare for potential questions you may have along the way.
Also as a best practice, I always have a running log of my key wins in each role I've had... There is so much that happens in the course of a few months or years that you tend to forget how much you've done and how much you've grown. Keep this list along with some work samples that you can use when asked.