First off, how old are you? Are you applying to programs straight out of undergrad? Also, silly question, but can you name examples of people who have succeeded in the fields that you are considering? I think it is worth noting that for every Madeline Albright or Condi Rice, there are 10000s of PhDs who perhaps are well-known in their respective niche fields, but have not become advisers or have succeeded in the "policy" world. Having worked with some individuals in these fields, many of them have a patron/mentor that they've been able to tag along with when they switch offices, jobs, etc. Sometimes, it is about timing and luck. Additionally, it depends what topic you are interested in with regards the PhD, as that will determine your ability to get job offers when you graduate. When I was debating these very same questions, I found these links very helpful:
http://www.cnas.org/blogs/abumuqawama/2011/08/phds-dummies.html
http://drezner.foreignpolicy.com/posts/2012/03/18/so_you_want_to_get_into_a_political_science_phd_program_part_one (Drezner has written a few posts on this topic)
Definitely read the comments, they are helpful as well.