Who to cite as referees for a post doc position?
This answer applies most directly to my field of mathematics, but it probably applies to similar fields as well.

Most genuine postdoc positions are extremely competitive. You need to apply well in advance (typically, in December for a position starting the next August). Postdocs will be difficult to obtain if your thesis work is substantially unfinished a year before you apply. I am not counting temporary teaching positions for new PhDs, which are a different kind of job entirely. 

For competitive postdocs, you want to have very strong letters, both from your advisor and from senior researchers at other institutions. The way to obtain such letters is to build connections throughout your graduate program, particularly in the later half. Ideally, you should plan to give one or two seminar talks at other institutions while in graduate school. In addition to disseminating your research, these talks help to get your name out as someone up and coming. Your advisor can also have a role, behind the scenes, in getting invitations. 

Competitive postdocs are given to people who seem to be up-and-coming in the field. There are usually only a handful of postdocs each year, worldwide, in each particular research specialty, so the committees have a collection of strong candidates to choose between, all of whom have the minimal credential of a PhD. So, if you want to pursue the postdoc track, your goal in graduate school is to develop a vita and letters that are competitive in that environment.